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Structural, mechanical, optical, dielectric and SHG studies of undoped and urea-doped γ-glycine crystals
ARTICLE IN PRESS Physica B 405 (2010) 738–743
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Structural, mechanical, optical, dielectric and SHG studies of undoped and urea-doped g-glycine crystals P. Selvarajan a,, J. Glorium Arulraj b, S. Perumal c a
Department of Physics, Aditanar College of Arts and Science, Tiruchendur 628216, Tamil Nadu, India Department of Physics, Nazareth Margoschis College, Nazareth 628617, Tamil Nadu, India c Physics Research Centre, S.T. Hindu College, Nagercoil 629003, Tamil Nadu, India b
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 April 2009 Received in revised form 14 September 2009 Accepted 30 September 2009
Single crystals of undoped and urea-doped g-glycine (gamma-glycine) were grown from aqueous solutions by slow evaporation technique. Morphological changes were noticed in g-glycine crystals when urea was added as dopant. Single crystal X-ray diffraction (XRD) studies were carried out to find crystal structure and lattice parameters of the grown crystals. UV-Visible transmittance spectra were recorded for the samples to analyze the transparency in visible and near infrared (NIR) region and UV cut-off wavelength observed for the samples to be at 257 nm. Nonlinear optical (NLO) activity of the grown crystals was studied using a Q-switched and pulsed Nd:YAG laser and second harmonic generation (SHG) efficiency was found. Values of work hardening coefficient were determined from microhardness studies and confirmed that the grown crystals belong to the category of soft materials. Measurements on values of dielectric constant, dielectric loss, AC conductivity and activation energy of the samples were carried out to understand the electrical phenomena that are taking place in pure and urea-doped g-glycine crystals. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn 61.10.Nz 81.40.Tv 77.22. d 42.70.Mp Keywords: Nonlinear optical materials Growth from solutions X-ray diffraction Crystal structure Doping Characterization methods Dielectric studies
1. Introduction Organic and semi-organic nonlinear optical (NLO) materials formed from amino acids have potential applications in second harmonic generation (SHG), optical storage, optical communication, photonics, electro-optic modulation, optical parametric amplifiers, optical image processing etc. [1–5]. It is known that glycine, the simplest and non-essential amino acid, exhibits in three different polymeric forms viz. a-glycine, b-glycine and g-glycine. Among the three forms, g-glycine exhibits strong piezoelectric and NLO effect [6–8]. Iitaka reported the details of g-glycine in 1958 and analyzed the crystal structure [9]. Growth of g-glycine crystals in the mixture of water with various sodium compounds was reported by Narayan Bhat et al. [10] and Srinivasan et al. has reported the growth of a-glycine and g-glycine and made detailed studies on the effect of sodium
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chloride (NaCl) on morphology of the grown crystals [11]. Recently, growth and various studies of g-glycine crystals have been reported by many authors and it is observed that g-glycine single crystals have been grown using additives such as potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium fluoride etc. [12–15]. In our work, g-glycine crystals have been grown by solution method using ammonium chloride as an additive. It has been reported that doping NLO crystals with organic impurities can alter various physical and chemical properties and doped-NLO crystals may find wide applications in opto-electronic devices compared to pure NLO crystals [16,17]. Since no work has been noticed in the literature on doped g-glycine crystals, an attempt has been made to introduce urea into the lattice of g-glycine crystal to alter its physical and chemical properties. Urea is a well known simple organic NLO material and if it is added as a dopant, it is expected to occupy the interstitial positions of the lattice, which may disturb the lattice of g-glycine crystal and in turn this may lead to alter the various properties g-glycine [18]. The aim of this paper is to report the growth and various studies such as UV-Visible-NIR transmittance
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studies, XRD studies, SHG, measurement of microhardness and hence work hardening coefficient, measurement of dielectric constant, loss, AC conductivity and activation energy of undoped and urea-doped g-glycine crystals.
2. Experimental methods 2.1. Synthesis and growth Initially, pure and urea-doped g-glycine salts were synthesized. Analar reagent (AR) grade of glycine and ammonium chloride in the molar ratio of 3:1 were used for synthesis of g-glycine salt. The calculated amounts of glycine and ammonium chloride were dissolved in de-ionized water and stirred well using a magnetic stirrer for about 2 h. The solution was heated until the synthesized salt of pure (undoped) g-glycine was obtained. To obtain ureadoped sample, 1 mol% of urea was added to the solution of g-glycine. Solubility of the synthesized salts of undoped and ureaadded g-glycine in de-ionized water at room temperature (30 1C) was found to be 22.5 and 23.8 g/100 ml, respectively. The method of finding solubility was already reported in the literature [19]. In accordance with the solubility data, saturated solutions of the synthesized salts of undoped and urea-doped g-glycine were prepared separately and the crystals were grown by solution method with slow solvent evaporation technique at room temperature (30 1C). The crystals were harvested after a period of 25–30 days.
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high transparency and large surface defect-free (i.e. without any pit or crack or scratch on the surface, tested with a traveling microscope) size were selected and used. The sample crystals were cut, polished and silver-electroded. The observations were made while cooling the sample. Since the area of the crystal was smaller than that of the plate area of the cell, the dielectric constant of the crystal was calculated using the relation C Cair ð1 Acrys =Aair Þ ðAair Þ er ¼ crys Cair ðAcrys Þ where Ccrys is the capacitance with crystal (including air), Cair is the capacitance of air, Acrys is the area of the crystal touching the electrode and Aair is the area of the electrode. Inaccuracy involved in the measurements of dielectric parameters was within 75%. AC electrical conductivity of the grown crystals was determined using the data available from dielectric measurements [19,22]. Second Harmonic Generation (SHG) test for the grown undoped and urea-doped g-glycine crystals was performed by the powder technique of Kurtz and Perry [23] using a pulsed Nd:YAG laser (Model: YG501C, l =1064 nm). Pulse energy of 4 mJ/pulse, pulse width of 10 ns and repetition rate of 10 Hz were used. The grown crystals were ground to powder of grain size 1500–1800 mm and the input laser beam was passed through IR reflector and directed on the powdered sample packed in a sample cell. Microcrystalline material of Potassium Dihydrogen Phosphate (KDP) was used as reference in this experiment. Second Harmonic Generation (SHG) from the samples was detected using a photomultiplier tube (PMT).
2.2. Characterization techniques 3. Results and discussion X-ray diffraction (XRD) provides an efficient and practical method for the structural characterization of crystals. This method helps in determining the arrangement and the spacing of atoms in a crystalline material. The grown single crystals of undoped and urea-doped g-glycine were subjected to single crystal X-ray diffraction (XRD) studies using an ENRAF NONIUS ˚ to CAD4 diffractometer with Mo Ka radiation (l =0.71073 A) identify the crystal structure, to find lattice parameters, space group and number of molecules per unit cell (Z). UV-Visible-NIR transmittance spectra of the samples were recorded using a Varian Cary 5E UV-Visible-NIR spectrophotometer in the range 200–1100 nm covering the near, visible, near infrared region to find the transmission range to know the suitability for optical applications. A crystal thickness of about 2 mm was used for transmission studies. The sample absorbs a portion of the incident radiation and the remainder is transmitted on to a detector. Mechanical property was studied by measuring microhardness of the grown crystals and this was carried out using Leitz Weitzler hardness tester fitted with a diamond indenter. Smooth, flat surface was selected and subjected to this study on the (1 0 0) plane of both undoped and urea-doped g-glycine crystals. Indentations were made for various loads from 10 to 65 g. Several trials of indentation were carried out on the (1 0 0) plane and the average diagonal lengths were measured for an indentation time of 10 s. The Vickers microhardness number was calculated using the relation Hv =1.8544P/d2 kg/mm2 where P is the applied load and d is the diagonal length of the indentation impression [20,21]. Measurements of dielectric parameters like capacitance, dielectric constant (er) and dielectric loss (tan d) of crystals carried out using an LCR meter (Agilent 4284A) at various frequencies in the range 102–106 Hz and at different temperatures ranging from 30 to 80 1C. Temperature was controlled to an accuracy of 70.1 1C and it was measured using a digital thermometer. Crystals with
3.1. Result of crystal growth The harvested single crystals of undoped and urea-doped
g-glycine grown by slow solvent evaporation technique are displayed in Fig. 1. The grown crystals are found to be polyhedron in shape and are stable, do not decompose in air and non-hygroscopic at ambient temperature. The grown crystals are observed to be transparent, colourless and the crystal faces and edges are well formed. The morphology of the crystal is found to be different when g-glycine is doped with urea and this is due to adsorption of the urea onto the surface of the crystal. The value of dipole moment of urea is 4.587 Debye units and this could influence of solvent-solute interactions in the solution. It is reported that urea acts as an immobile impurity that is usually adsorbed at the terrace of the crystal during the growth. Adsorption of urea on the surface of the crystal take place during the growth and hence the dopants have been introduced into the lattice of g-glycine crystal. It is possible that adsorption of
Fig. 1. Harvested crystals of (a) pure and (b) urea-doped g-glycine crystals.
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dopant (urea) may lead to change of morphology of the doped crystal [24,25]. Since the synthesized salt of urea-doped g-glycine was used to grow the doped crystals, the approximate amount present in the urea-doped g-glycine crystals may be considered to be of about 1 mol%. As urea is an organic material, it is very difficult to find the exact amount of urea present in the doped crystal quantitatively. Here it is to be noted that the changes in the values of physical properties such as microhardness, UV-visible transmittance, lattice constants, dielectric constant, conductivity etc reveal the presence of the dopants (urea) in the doped crystals qualitatively. 3.2. Single crystal XRD studies Single crystal XRD data of undoped and urea-doped g-glycine crystals were collected from a single crystal X-ray diffractometer with graphite-monochromated Mo Ka radiation. From the data, it is observed that the grown crystals crystallize in hexagonal or trigonal system with the space group P32. The number of molecules per unit cell (Z) for both crystals of this work is found ˚ to be 3. The lattice parameters are found to be a = 7.038(2) A, ˚ ˚ c=5.484(1) A, ˚ a =901, b =901, g =1201 and a=7.013(1) A, b=7.038(2) A, ˚ c =5.614(2) A, ˚ a = 901, b =901, g =1201 for pure and b= 7.013(1) A, urea-doped g-glycine crystals, respectively. It is reported that a-glycine and b-glycine crystals crystallize in centrosymmetric space group where as g-glycine crystallizes in non-centrosymmetric space group and hence g-glycine is an NLO active material. A g-glycine molecule contains a proton donor carboxyl group and a proton acceptor amino group and a network of hydrogen bonds holds the amino nitrogen and carboxyl oxygen in the crystal and this leads to the formation of a non-centrosymmetric space group in the g-glycine crystal. The unit cell parameters of undoped g-glycine obtained in this work are very close agreement with reported work [26] and slight changes of lattice parameters have been noticed for the urea-doped sample compared to undoped one. The changes in the lattice parameters are due to incorporation of urea in the lattice of g-glycine crystal. The presence of dopants in g-glycine crystal may produce lattice strain which leads to change of unit cell parameters in the urea-doped sample. 3.3. Transmittance studies Optical transmittance spectra of undoped and urea-doped
g-glycine crystals in the wavelength range 200–1100 nm are shown in Fig. 2. This spectral study may be assisted in
100 1 2
Transmittance (%)
80 60
(1) Undoped gamma-glycine crystal (2) Urea-doped gamma-glycine
understanding electronic structure of the optical band gap of the crystal. The study of the absorption edge is essential in connection with the theory of electronic structure, which leads to the prediction of whether the band structure is affected near the band extreme. From the transmittance spectra, it is noticed that undoped and urea-doped g-glycine crystals have high transmittance in the entire visible-NIR region of the spectra and the high transmission in the entire visible region and short cut-off wavelength facilitates the grown crystals of this work to be potential nonlinear optical materials for second harmonic and third harmonic of Nd:YAG laser. A sharp fall in the transmittance is observed at 257 nm for both pure and urea-doped g-glycine crystals and this corresponds to the fundamental absorption (UV cut-off wavelength). It is observed that doping g-glycine crystal with urea reduces the percentage of transmission and does not alter the lower cut-off wavelength. Absorption in the near ultraviolet region arises from electronic transitions associated within the samples. Using the formula Eg = 1240/l (nm), the band gap is calculated to be 4.824 eV for both pure and urea-doped g-glycine crystals. The observed behaviour of the spectra and band gap value found in this work are in good agreement with spectral data of g-glycine crystal reported in literature [12]. 3.4. Microhardness studies Mechanical strength of pure (undoped) and urea-doped
g-glycine crystals was studied by measuring microhardness and it plays an important role in the fabrication of opto-electronic devices. The hardness of a material is a measure of its resistance to plastic deformation. The permanent deformation can be achieved by indentation, bending, scratching or cutting. In an ideal crystal, the hardness value should be independent of applied load. But in a real crystal, the load dependence is observed. This is due to normal indentation size effect (ISE) [27]. The values of microhardness number at different loads applied to the samples for the plane (1 0 0) are provided in the Table 1 and it is noticed that Vickers hardness number (Hv) decreases with the applied load satisfying the indentation size effect. The observed values from microhardness studies of undoped g-glycine crystal in the present study is observed to be same as in the literature [28]. From the results, it is observed that the hardness of g-glycine crystal increases when it is doped with urea. This increase in the hardness value of doped sample can be attributed to the incorporation of impurity (urea) in the lattice of g-glycine crystal. The addition of urea to g-glycine crystalline sample most probably enhances the strength of bonding with the host material and hence hardness number increases. The relationship between load (P) and diagonal length (d) of indentation is given by P= adn, which is known as Mayer’s law [29]. Here a and n are constants for a particular material. From straight line graph of log d versus log P (Fig. 3), the reciprocal of slope can be obtained and it is equal to the constant n which is Table 1 Values of microhardness number at different loads for pure and urea-doped gglycine crystals.
40 20
Load (g)
Pure g-glycine crystal 2
Urea-doped g-glycine
Hv (kg/mm )
Hv (kg/mm2)
62.5 54.7 47.6 43.8 40.9 38.3
71.8 64.9 59.3 50.5 48.2 45.4
0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 2. Transmittance spectra of pure and urea-doped g-glycine crystals.
10 20 30 40 50 60
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Pure γ-glycine crystal Urea-doped γ-glycine crystal
1.7
Dielectric constant
Log d
1.6 1.5 1.4 1.3
741
150
at 30 °C
140
at 50 °C
130
at 80 °C
120 110 100 90 80 70 60
1.2
2 1.2
1.4 Log P
1.6
3
1.8
Fig. 3. Plots of Log d versus Log P for pure and urea-doped g-glycine crystals.
equal to Mayer’s index number or work hardening coefficient. The values of n obtained from the graph are 1.71 and 1.87 for pure and urea-doped g-glycine crystals, respectively. According to Onitsch’s theory, if n is greater than 1.6, the materials are said to be soft materials [30]. Hence the grown crystals of this work belong to the category of soft materials. Since g-glycine crystals find application in opto-electronic and communication devices, the increase in the hardness of urea-doped g-glycine crystals will have a significant effect on the fabrication and processing of devices.
4 Log frequency
5
6
Fig. 4. Variation of dielectric constant of pure g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
Dielectric constant
1.0
3.5. Dielectric studies
150
at 30 °C
140
at 50 °C at 80 °C
130 120 110 100 90 80 70 2
3
4 Log frequency
5
6
Fig. 5. Variation of dielectric constant of urea-doped g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
0.9 at 30 °C
0.8
at 50 °C
0.7 Dielectric loss
The dielectric parameters like dielectric constant (er) and dielectric loss (tan d) are the basic electrical properties of solids. The measurement of dielectric constant and loss as a function of frequency and temperature gives the ideas of electrical processes that are taking place in materials and these parameters were measured on (1 0 0) face of the samples of this work. Variation of dielectric constant and dielectric loss of pure and urea-doped g-glycine crystals as a function log frequency at temperatures 30, 50 and 80 1C are displayed in the Figs. 4–7. Since frequency values vary from 102–106 Hz, the scale will not be equal if log frequency is not taken along X-axis in the Figs. 4–7. The obtained results suggest that the dielectric constant and loss strongly depend on the frequency of applied field and the temperature of the samples. It is observed from the figures that both dielectric constant and loss are high at low frequencies and they decrease with increase in frequency and attain almost constant values beyond 104 Hz. It is noticed that the values of er and tan d are more for urea-doped g-glycine crystal when compared to those of undoped sample in the considered frequency range and temperature range of this work. When temperature is increased, the dielectric parameters are found to be increased for both the samples in the whole frequency range (102–106 Hz). Recollecting our data, the high values of dielectric constant (er) and dielectric loss (tan d) of pure and g-glycine crystals at low frequencies are ascribed to space charge polarization due to charged lattice defects. It is to be noted here that space charge polarization is dominant and electronic and ionic polarizations are not very much active in low frequency region. The nature of decrease of er and tan d with frequency suggests that the crystals of this work seem to contain dipoles of continuously varying relaxation times. Since the dipoles of larger relaxation times are not able to respond to the higher frequencies, the dielectric constant and loss tangent are low at higher frequencies [31,32].
at 80 °C
0.6 0.5 0.4 0.3 0.2 0.1 2
3
4 Log frequency
5
6
Fig. 6. Variation of dielectric loss of pure g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
When urea is added as the dopant, it is possible that urea may occupy the interstitial positions of lattice of g-glycine crystal and the presence of defects (dopants) may be responsible for increase in the space charge polarization and hence there is an
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Dielectric loss
1.1 1.0
at 30 °C
0.9
at 50 °C
0.8
at 80 °C
0.7 0.6 0.5 0.4 0.3 0.2 0.1 2
3
4 Log frequency
5
6
Fig. 7. Variation of dielectric loss of urea-doped g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
Pure γ −glycine crystal Urea-doped γ -glycine crystal
-12.0
ln σac
-12.5 -13.0 -13.5 -14.0 -14.5 2.8
2.9
3.0
3.1
3.2
energy required for charge carriers to take part in the conduction process of the samples. A graph of ln sac versus 1/T is depicted in the Fig. 8 and the activation energy was found to be 0.451 and 0.392 eV for pure and urea-doped g-glycine crystals, respectively. It is observed that conductivity increases with temperature for both the samples and conductivity is more and activation energy is less for urea-doped sample than that for undoped g-glycine sample. The decrease in activation energy may be due to the presence of dopants (urea) in the doped-sample. When g-glycine crystal is doped with urea, more lattice defects will be present in the doped-sample and this leads to increase in the value of conductivity and hence there is a decrease of activation energy [17]. g-glycine is an organic crystal and in such a system, the dielectric parameters like dielectric constant, dielectric loss and AC conductivity also depends upon electron-phonon interactions between the defects and the phonon subsystem [36]. 3.6. Second harmonic generation (SHG) test In order to confirm nonlinear optical (NLO) property, microcrystalline form of pure and urea-doped g-glycine crystals were packed separately between two transparent glass slides (sample cell). A fundamental laser beam of 1064 nm from a Nd:YAG laser was made to fall on the sample cell and Second Harmonic Generation (SHG) was confirmed by emission of green light (l =532 nm). The measured second harmonic conversion efficiency of undoped and urea-doped g-glycine samples was about 1.55 and 1.61 times as that of KDP. The measured value of SHG efficiency for the pure sample is found to be in agreement with reported literature [10]. When g-glycine crystal is doped with urea, the SHG efficiency is observed to be slightly enhanced and this may be due to incorporation of urea as the dopant in the lattice of the host crystal. The electron-phonon interactions between the defects (urea) and the phonon subsystem are also responsible for the enhancement of the SHG efficiency in the case of urea-doped g-glycine crystal [36].
3.3
1000/T (K-1) Fig. 8. Plots of ln sac versus 1/T at frequency of 103 Hz for pure and urea-doped gglycine crystals.
enhancement of dielectric constant and loss values in the case of urea-doped sample in the low frequency region. Variation of the dielectric parameters with temperature is generally attributed to the crystal expansion, the electronic, space charge and ionic polarizations and the presence of impurities and crystal defects. The increase in the values of dielectric parameters at higher temperatures is mainly attributed to the thermally generated charge carriers and impurity dipoles. As far as polarization is concerned, the increase in dielectric constant with temperature is essentially due to the temperature variation of ionic and space charge polarizations and not due to the temperature variation of orientational polarization [33]. AC conductivity of the samples was determined using the relation sac = 2pfeoer tan d where f is the frequency of AC. signal, eo is the permittivity of free space or vacuum, er is the dielectric constant and tan d is the dielectric loss of the sample [34,35]. Since the samples are insulating materials, the values of AC conductivity (sac) are fitted in the equation sac = s exp( Eac/kBT) and the activation energy (Eac) was calculated. Here kB is the Boltzmann’s constant, T is the absolute temperature, s is a constant depending on the material. Activation energy is the
4. Conclusions Slow solvent evaporation technique was adopted to grow single crystals of undoped and urea-doped g-glycine in the period of 25–30 days. The grown crystals are observed to be transparent and colourless with well-defined external appearance. When urea is added as dopant, the morphology of g-glycine crystals is altered. Single crystal XRD studies confirm that the grown crystals of this work crystallize in non-centrosymmetric space group and crystal structure of g-glycine crystal is not changed when it is doped with urea. The powder SHG test confirms the NLO property of undoped and urea-doped g-glycine crystals and these materials are suitable for NLO applications. In the case of urea-doped g-glycine crystal, the SHG efficiency is observed to be enhanced. Microhardness study indicates that urea-doped g-glycine crystal has more mechanical strength than that of undoped g-glycine crystal and from the data of microhardness studies, work hardening coefficient of the crystals was determined. The UVVisible-NIR spectra show that the grown crystals have good optical transmittance window in the entire visible-NIR region. Dielectric studies were carried out for pure and urea-doped g-glycine crystals and dielectric constant and loss tangent are found to be decreasing with increase of frequency and increasing with increase of temperature. It is observed that dielectric constant, dielectric loss and hence AC conductivity are more for urea-doped g-glycine crystal than those of undoped one. Activation energy of the samples was determined and it is observed
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to be less for urea-doped sample when compared to undoped g-glycine crystal. Acknowledgements The supports extended in the research by Sophisticated Analytical Facility (SAIF), IIT, Chennai, IISc, Bangalore and M.K.University, Madurai are gratefully acknowledged. We thank authorities of Aditanar College of Arts and Science, Tiruchendur and S.T. Hindu College, Nagercoil for the encouragement to carry out the research work. References [1] K. Meera, R. Muralidharan, R. Dhanasekaran, P. Manyum, P. Ramasamy, J. Crystal Growth 263 (2004) 510. [2] S. Aruna, G. Bhagavannarayana, M. Palanisamy, P.C. Thomas, B. Varghese, P. Sagayaraj, J. Crystal Growth 300 (2007) 403. [3] T. Pal, T. Kar, Mater. Chem. Phys. 91 (2005) 343. [4] S.B. Monaco, L.E. Davis, S.P. Velsko, F.T. Wang, D. Eimerl, A.J. Zalkin, J. Crystal Growth 85 (1987) 252. [5] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Optics Comm. 281 (2008) 2285. [6] G. Albrecht, R.B. Corey, J. Amer. Chem. Soc. 61 (1939) 1087. [7] E. Ramachandran, K. Baskaran, S. Natarajan, Cryst. Res. Tech. 42 (2007) 73. [8] B. Narayana Moolya, A. Jayarama, M.R. Sureshkumar, S.M. Dharmaprakash, J. Crystal Growth 280 (2005) 581. [9] Y. Iitaka, Acta Cryst. 11 (1958) 225. [10] M. Narayan Bhat, S.M. Dharmaprakash, J. Crystal Growth 236 (2002) 376.
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Structural, mechanical, optical, dielectric and SHG studies of undoped and urea-doped g-glycine crystals P. Selvarajan a,, J. Glorium Arulraj b, S. Perumal c a
Department of Physics, Aditanar College of Arts and Science, Tiruchendur 628216, Tamil Nadu, India Department of Physics, Nazareth Margoschis College, Nazareth 628617, Tamil Nadu, India c Physics Research Centre, S.T. Hindu College, Nagercoil 629003, Tamil Nadu, India b
a r t i c l e in f o
a b s t r a c t
Article history: Received 25 April 2009 Received in revised form 14 September 2009 Accepted 30 September 2009
Single crystals of undoped and urea-doped g-glycine (gamma-glycine) were grown from aqueous solutions by slow evaporation technique. Morphological changes were noticed in g-glycine crystals when urea was added as dopant. Single crystal X-ray diffraction (XRD) studies were carried out to find crystal structure and lattice parameters of the grown crystals. UV-Visible transmittance spectra were recorded for the samples to analyze the transparency in visible and near infrared (NIR) region and UV cut-off wavelength observed for the samples to be at 257 nm. Nonlinear optical (NLO) activity of the grown crystals was studied using a Q-switched and pulsed Nd:YAG laser and second harmonic generation (SHG) efficiency was found. Values of work hardening coefficient were determined from microhardness studies and confirmed that the grown crystals belong to the category of soft materials. Measurements on values of dielectric constant, dielectric loss, AC conductivity and activation energy of the samples were carried out to understand the electrical phenomena that are taking place in pure and urea-doped g-glycine crystals. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.10.Dn 61.10.Nz 81.40.Tv 77.22. d 42.70.Mp Keywords: Nonlinear optical materials Growth from solutions X-ray diffraction Crystal structure Doping Characterization methods Dielectric studies
1. Introduction Organic and semi-organic nonlinear optical (NLO) materials formed from amino acids have potential applications in second harmonic generation (SHG), optical storage, optical communication, photonics, electro-optic modulation, optical parametric amplifiers, optical image processing etc. [1–5]. It is known that glycine, the simplest and non-essential amino acid, exhibits in three different polymeric forms viz. a-glycine, b-glycine and g-glycine. Among the three forms, g-glycine exhibits strong piezoelectric and NLO effect [6–8]. Iitaka reported the details of g-glycine in 1958 and analyzed the crystal structure [9]. Growth of g-glycine crystals in the mixture of water with various sodium compounds was reported by Narayan Bhat et al. [10] and Srinivasan et al. has reported the growth of a-glycine and g-glycine and made detailed studies on the effect of sodium
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chloride (NaCl) on morphology of the grown crystals [11]. Recently, growth and various studies of g-glycine crystals have been reported by many authors and it is observed that g-glycine single crystals have been grown using additives such as potassium chloride, sodium chloride, lithium chloride, potassium bromide, sodium fluoride etc. [12–15]. In our work, g-glycine crystals have been grown by solution method using ammonium chloride as an additive. It has been reported that doping NLO crystals with organic impurities can alter various physical and chemical properties and doped-NLO crystals may find wide applications in opto-electronic devices compared to pure NLO crystals [16,17]. Since no work has been noticed in the literature on doped g-glycine crystals, an attempt has been made to introduce urea into the lattice of g-glycine crystal to alter its physical and chemical properties. Urea is a well known simple organic NLO material and if it is added as a dopant, it is expected to occupy the interstitial positions of the lattice, which may disturb the lattice of g-glycine crystal and in turn this may lead to alter the various properties g-glycine [18]. The aim of this paper is to report the growth and various studies such as UV-Visible-NIR transmittance
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studies, XRD studies, SHG, measurement of microhardness and hence work hardening coefficient, measurement of dielectric constant, loss, AC conductivity and activation energy of undoped and urea-doped g-glycine crystals.
2. Experimental methods 2.1. Synthesis and growth Initially, pure and urea-doped g-glycine salts were synthesized. Analar reagent (AR) grade of glycine and ammonium chloride in the molar ratio of 3:1 were used for synthesis of g-glycine salt. The calculated amounts of glycine and ammonium chloride were dissolved in de-ionized water and stirred well using a magnetic stirrer for about 2 h. The solution was heated until the synthesized salt of pure (undoped) g-glycine was obtained. To obtain ureadoped sample, 1 mol% of urea was added to the solution of g-glycine. Solubility of the synthesized salts of undoped and ureaadded g-glycine in de-ionized water at room temperature (30 1C) was found to be 22.5 and 23.8 g/100 ml, respectively. The method of finding solubility was already reported in the literature [19]. In accordance with the solubility data, saturated solutions of the synthesized salts of undoped and urea-doped g-glycine were prepared separately and the crystals were grown by solution method with slow solvent evaporation technique at room temperature (30 1C). The crystals were harvested after a period of 25–30 days.
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high transparency and large surface defect-free (i.e. without any pit or crack or scratch on the surface, tested with a traveling microscope) size were selected and used. The sample crystals were cut, polished and silver-electroded. The observations were made while cooling the sample. Since the area of the crystal was smaller than that of the plate area of the cell, the dielectric constant of the crystal was calculated using the relation C Cair ð1 Acrys =Aair Þ ðAair Þ er ¼ crys Cair ðAcrys Þ where Ccrys is the capacitance with crystal (including air), Cair is the capacitance of air, Acrys is the area of the crystal touching the electrode and Aair is the area of the electrode. Inaccuracy involved in the measurements of dielectric parameters was within 75%. AC electrical conductivity of the grown crystals was determined using the data available from dielectric measurements [19,22]. Second Harmonic Generation (SHG) test for the grown undoped and urea-doped g-glycine crystals was performed by the powder technique of Kurtz and Perry [23] using a pulsed Nd:YAG laser (Model: YG501C, l =1064 nm). Pulse energy of 4 mJ/pulse, pulse width of 10 ns and repetition rate of 10 Hz were used. The grown crystals were ground to powder of grain size 1500–1800 mm and the input laser beam was passed through IR reflector and directed on the powdered sample packed in a sample cell. Microcrystalline material of Potassium Dihydrogen Phosphate (KDP) was used as reference in this experiment. Second Harmonic Generation (SHG) from the samples was detected using a photomultiplier tube (PMT).
2.2. Characterization techniques 3. Results and discussion X-ray diffraction (XRD) provides an efficient and practical method for the structural characterization of crystals. This method helps in determining the arrangement and the spacing of atoms in a crystalline material. The grown single crystals of undoped and urea-doped g-glycine were subjected to single crystal X-ray diffraction (XRD) studies using an ENRAF NONIUS ˚ to CAD4 diffractometer with Mo Ka radiation (l =0.71073 A) identify the crystal structure, to find lattice parameters, space group and number of molecules per unit cell (Z). UV-Visible-NIR transmittance spectra of the samples were recorded using a Varian Cary 5E UV-Visible-NIR spectrophotometer in the range 200–1100 nm covering the near, visible, near infrared region to find the transmission range to know the suitability for optical applications. A crystal thickness of about 2 mm was used for transmission studies. The sample absorbs a portion of the incident radiation and the remainder is transmitted on to a detector. Mechanical property was studied by measuring microhardness of the grown crystals and this was carried out using Leitz Weitzler hardness tester fitted with a diamond indenter. Smooth, flat surface was selected and subjected to this study on the (1 0 0) plane of both undoped and urea-doped g-glycine crystals. Indentations were made for various loads from 10 to 65 g. Several trials of indentation were carried out on the (1 0 0) plane and the average diagonal lengths were measured for an indentation time of 10 s. The Vickers microhardness number was calculated using the relation Hv =1.8544P/d2 kg/mm2 where P is the applied load and d is the diagonal length of the indentation impression [20,21]. Measurements of dielectric parameters like capacitance, dielectric constant (er) and dielectric loss (tan d) of crystals carried out using an LCR meter (Agilent 4284A) at various frequencies in the range 102–106 Hz and at different temperatures ranging from 30 to 80 1C. Temperature was controlled to an accuracy of 70.1 1C and it was measured using a digital thermometer. Crystals with
3.1. Result of crystal growth The harvested single crystals of undoped and urea-doped
g-glycine grown by slow solvent evaporation technique are displayed in Fig. 1. The grown crystals are found to be polyhedron in shape and are stable, do not decompose in air and non-hygroscopic at ambient temperature. The grown crystals are observed to be transparent, colourless and the crystal faces and edges are well formed. The morphology of the crystal is found to be different when g-glycine is doped with urea and this is due to adsorption of the urea onto the surface of the crystal. The value of dipole moment of urea is 4.587 Debye units and this could influence of solvent-solute interactions in the solution. It is reported that urea acts as an immobile impurity that is usually adsorbed at the terrace of the crystal during the growth. Adsorption of urea on the surface of the crystal take place during the growth and hence the dopants have been introduced into the lattice of g-glycine crystal. It is possible that adsorption of
Fig. 1. Harvested crystals of (a) pure and (b) urea-doped g-glycine crystals.
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dopant (urea) may lead to change of morphology of the doped crystal [24,25]. Since the synthesized salt of urea-doped g-glycine was used to grow the doped crystals, the approximate amount present in the urea-doped g-glycine crystals may be considered to be of about 1 mol%. As urea is an organic material, it is very difficult to find the exact amount of urea present in the doped crystal quantitatively. Here it is to be noted that the changes in the values of physical properties such as microhardness, UV-visible transmittance, lattice constants, dielectric constant, conductivity etc reveal the presence of the dopants (urea) in the doped crystals qualitatively. 3.2. Single crystal XRD studies Single crystal XRD data of undoped and urea-doped g-glycine crystals were collected from a single crystal X-ray diffractometer with graphite-monochromated Mo Ka radiation. From the data, it is observed that the grown crystals crystallize in hexagonal or trigonal system with the space group P32. The number of molecules per unit cell (Z) for both crystals of this work is found ˚ to be 3. The lattice parameters are found to be a = 7.038(2) A, ˚ ˚ c=5.484(1) A, ˚ a =901, b =901, g =1201 and a=7.013(1) A, b=7.038(2) A, ˚ c =5.614(2) A, ˚ a = 901, b =901, g =1201 for pure and b= 7.013(1) A, urea-doped g-glycine crystals, respectively. It is reported that a-glycine and b-glycine crystals crystallize in centrosymmetric space group where as g-glycine crystallizes in non-centrosymmetric space group and hence g-glycine is an NLO active material. A g-glycine molecule contains a proton donor carboxyl group and a proton acceptor amino group and a network of hydrogen bonds holds the amino nitrogen and carboxyl oxygen in the crystal and this leads to the formation of a non-centrosymmetric space group in the g-glycine crystal. The unit cell parameters of undoped g-glycine obtained in this work are very close agreement with reported work [26] and slight changes of lattice parameters have been noticed for the urea-doped sample compared to undoped one. The changes in the lattice parameters are due to incorporation of urea in the lattice of g-glycine crystal. The presence of dopants in g-glycine crystal may produce lattice strain which leads to change of unit cell parameters in the urea-doped sample. 3.3. Transmittance studies Optical transmittance spectra of undoped and urea-doped
g-glycine crystals in the wavelength range 200–1100 nm are shown in Fig. 2. This spectral study may be assisted in
100 1 2
Transmittance (%)
80 60
(1) Undoped gamma-glycine crystal (2) Urea-doped gamma-glycine
understanding electronic structure of the optical band gap of the crystal. The study of the absorption edge is essential in connection with the theory of electronic structure, which leads to the prediction of whether the band structure is affected near the band extreme. From the transmittance spectra, it is noticed that undoped and urea-doped g-glycine crystals have high transmittance in the entire visible-NIR region of the spectra and the high transmission in the entire visible region and short cut-off wavelength facilitates the grown crystals of this work to be potential nonlinear optical materials for second harmonic and third harmonic of Nd:YAG laser. A sharp fall in the transmittance is observed at 257 nm for both pure and urea-doped g-glycine crystals and this corresponds to the fundamental absorption (UV cut-off wavelength). It is observed that doping g-glycine crystal with urea reduces the percentage of transmission and does not alter the lower cut-off wavelength. Absorption in the near ultraviolet region arises from electronic transitions associated within the samples. Using the formula Eg = 1240/l (nm), the band gap is calculated to be 4.824 eV for both pure and urea-doped g-glycine crystals. The observed behaviour of the spectra and band gap value found in this work are in good agreement with spectral data of g-glycine crystal reported in literature [12]. 3.4. Microhardness studies Mechanical strength of pure (undoped) and urea-doped
g-glycine crystals was studied by measuring microhardness and it plays an important role in the fabrication of opto-electronic devices. The hardness of a material is a measure of its resistance to plastic deformation. The permanent deformation can be achieved by indentation, bending, scratching or cutting. In an ideal crystal, the hardness value should be independent of applied load. But in a real crystal, the load dependence is observed. This is due to normal indentation size effect (ISE) [27]. The values of microhardness number at different loads applied to the samples for the plane (1 0 0) are provided in the Table 1 and it is noticed that Vickers hardness number (Hv) decreases with the applied load satisfying the indentation size effect. The observed values from microhardness studies of undoped g-glycine crystal in the present study is observed to be same as in the literature [28]. From the results, it is observed that the hardness of g-glycine crystal increases when it is doped with urea. This increase in the hardness value of doped sample can be attributed to the incorporation of impurity (urea) in the lattice of g-glycine crystal. The addition of urea to g-glycine crystalline sample most probably enhances the strength of bonding with the host material and hence hardness number increases. The relationship between load (P) and diagonal length (d) of indentation is given by P= adn, which is known as Mayer’s law [29]. Here a and n are constants for a particular material. From straight line graph of log d versus log P (Fig. 3), the reciprocal of slope can be obtained and it is equal to the constant n which is Table 1 Values of microhardness number at different loads for pure and urea-doped gglycine crystals.
40 20
Load (g)
Pure g-glycine crystal 2
Urea-doped g-glycine
Hv (kg/mm )
Hv (kg/mm2)
62.5 54.7 47.6 43.8 40.9 38.3
71.8 64.9 59.3 50.5 48.2 45.4
0 200
400
600
800
1000
1200
Wavelength (nm) Fig. 2. Transmittance spectra of pure and urea-doped g-glycine crystals.
10 20 30 40 50 60
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Pure γ-glycine crystal Urea-doped γ-glycine crystal
1.7
Dielectric constant
Log d
1.6 1.5 1.4 1.3
741
150
at 30 °C
140
at 50 °C
130
at 80 °C
120 110 100 90 80 70 60
1.2
2 1.2
1.4 Log P
1.6
3
1.8
Fig. 3. Plots of Log d versus Log P for pure and urea-doped g-glycine crystals.
equal to Mayer’s index number or work hardening coefficient. The values of n obtained from the graph are 1.71 and 1.87 for pure and urea-doped g-glycine crystals, respectively. According to Onitsch’s theory, if n is greater than 1.6, the materials are said to be soft materials [30]. Hence the grown crystals of this work belong to the category of soft materials. Since g-glycine crystals find application in opto-electronic and communication devices, the increase in the hardness of urea-doped g-glycine crystals will have a significant effect on the fabrication and processing of devices.
4 Log frequency
5
6
Fig. 4. Variation of dielectric constant of pure g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
Dielectric constant
1.0
3.5. Dielectric studies
150
at 30 °C
140
at 50 °C at 80 °C
130 120 110 100 90 80 70 2
3
4 Log frequency
5
6
Fig. 5. Variation of dielectric constant of urea-doped g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
0.9 at 30 °C
0.8
at 50 °C
0.7 Dielectric loss
The dielectric parameters like dielectric constant (er) and dielectric loss (tan d) are the basic electrical properties of solids. The measurement of dielectric constant and loss as a function of frequency and temperature gives the ideas of electrical processes that are taking place in materials and these parameters were measured on (1 0 0) face of the samples of this work. Variation of dielectric constant and dielectric loss of pure and urea-doped g-glycine crystals as a function log frequency at temperatures 30, 50 and 80 1C are displayed in the Figs. 4–7. Since frequency values vary from 102–106 Hz, the scale will not be equal if log frequency is not taken along X-axis in the Figs. 4–7. The obtained results suggest that the dielectric constant and loss strongly depend on the frequency of applied field and the temperature of the samples. It is observed from the figures that both dielectric constant and loss are high at low frequencies and they decrease with increase in frequency and attain almost constant values beyond 104 Hz. It is noticed that the values of er and tan d are more for urea-doped g-glycine crystal when compared to those of undoped sample in the considered frequency range and temperature range of this work. When temperature is increased, the dielectric parameters are found to be increased for both the samples in the whole frequency range (102–106 Hz). Recollecting our data, the high values of dielectric constant (er) and dielectric loss (tan d) of pure and g-glycine crystals at low frequencies are ascribed to space charge polarization due to charged lattice defects. It is to be noted here that space charge polarization is dominant and electronic and ionic polarizations are not very much active in low frequency region. The nature of decrease of er and tan d with frequency suggests that the crystals of this work seem to contain dipoles of continuously varying relaxation times. Since the dipoles of larger relaxation times are not able to respond to the higher frequencies, the dielectric constant and loss tangent are low at higher frequencies [31,32].
at 80 °C
0.6 0.5 0.4 0.3 0.2 0.1 2
3
4 Log frequency
5
6
Fig. 6. Variation of dielectric loss of pure g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
When urea is added as the dopant, it is possible that urea may occupy the interstitial positions of lattice of g-glycine crystal and the presence of defects (dopants) may be responsible for increase in the space charge polarization and hence there is an
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Dielectric loss
1.1 1.0
at 30 °C
0.9
at 50 °C
0.8
at 80 °C
0.7 0.6 0.5 0.4 0.3 0.2 0.1 2
3
4 Log frequency
5
6
Fig. 7. Variation of dielectric loss of urea-doped g-glycine crystal with frequency at temperatures 30, 50 and 80 1C.
Pure γ −glycine crystal Urea-doped γ -glycine crystal
-12.0
ln σac
-12.5 -13.0 -13.5 -14.0 -14.5 2.8
2.9
3.0
3.1
3.2
energy required for charge carriers to take part in the conduction process of the samples. A graph of ln sac versus 1/T is depicted in the Fig. 8 and the activation energy was found to be 0.451 and 0.392 eV for pure and urea-doped g-glycine crystals, respectively. It is observed that conductivity increases with temperature for both the samples and conductivity is more and activation energy is less for urea-doped sample than that for undoped g-glycine sample. The decrease in activation energy may be due to the presence of dopants (urea) in the doped-sample. When g-glycine crystal is doped with urea, more lattice defects will be present in the doped-sample and this leads to increase in the value of conductivity and hence there is a decrease of activation energy [17]. g-glycine is an organic crystal and in such a system, the dielectric parameters like dielectric constant, dielectric loss and AC conductivity also depends upon electron-phonon interactions between the defects and the phonon subsystem [36]. 3.6. Second harmonic generation (SHG) test In order to confirm nonlinear optical (NLO) property, microcrystalline form of pure and urea-doped g-glycine crystals were packed separately between two transparent glass slides (sample cell). A fundamental laser beam of 1064 nm from a Nd:YAG laser was made to fall on the sample cell and Second Harmonic Generation (SHG) was confirmed by emission of green light (l =532 nm). The measured second harmonic conversion efficiency of undoped and urea-doped g-glycine samples was about 1.55 and 1.61 times as that of KDP. The measured value of SHG efficiency for the pure sample is found to be in agreement with reported literature [10]. When g-glycine crystal is doped with urea, the SHG efficiency is observed to be slightly enhanced and this may be due to incorporation of urea as the dopant in the lattice of the host crystal. The electron-phonon interactions between the defects (urea) and the phonon subsystem are also responsible for the enhancement of the SHG efficiency in the case of urea-doped g-glycine crystal [36].
3.3
1000/T (K-1) Fig. 8. Plots of ln sac versus 1/T at frequency of 103 Hz for pure and urea-doped gglycine crystals.
enhancement of dielectric constant and loss values in the case of urea-doped sample in the low frequency region. Variation of the dielectric parameters with temperature is generally attributed to the crystal expansion, the electronic, space charge and ionic polarizations and the presence of impurities and crystal defects. The increase in the values of dielectric parameters at higher temperatures is mainly attributed to the thermally generated charge carriers and impurity dipoles. As far as polarization is concerned, the increase in dielectric constant with temperature is essentially due to the temperature variation of ionic and space charge polarizations and not due to the temperature variation of orientational polarization [33]. AC conductivity of the samples was determined using the relation sac = 2pfeoer tan d where f is the frequency of AC. signal, eo is the permittivity of free space or vacuum, er is the dielectric constant and tan d is the dielectric loss of the sample [34,35]. Since the samples are insulating materials, the values of AC conductivity (sac) are fitted in the equation sac = s exp( Eac/kBT) and the activation energy (Eac) was calculated. Here kB is the Boltzmann’s constant, T is the absolute temperature, s is a constant depending on the material. Activation energy is the
4. Conclusions Slow solvent evaporation technique was adopted to grow single crystals of undoped and urea-doped g-glycine in the period of 25–30 days. The grown crystals are observed to be transparent and colourless with well-defined external appearance. When urea is added as dopant, the morphology of g-glycine crystals is altered. Single crystal XRD studies confirm that the grown crystals of this work crystallize in non-centrosymmetric space group and crystal structure of g-glycine crystal is not changed when it is doped with urea. The powder SHG test confirms the NLO property of undoped and urea-doped g-glycine crystals and these materials are suitable for NLO applications. In the case of urea-doped g-glycine crystal, the SHG efficiency is observed to be enhanced. Microhardness study indicates that urea-doped g-glycine crystal has more mechanical strength than that of undoped g-glycine crystal and from the data of microhardness studies, work hardening coefficient of the crystals was determined. The UVVisible-NIR spectra show that the grown crystals have good optical transmittance window in the entire visible-NIR region. Dielectric studies were carried out for pure and urea-doped g-glycine crystals and dielectric constant and loss tangent are found to be decreasing with increase of frequency and increasing with increase of temperature. It is observed that dielectric constant, dielectric loss and hence AC conductivity are more for urea-doped g-glycine crystal than those of undoped one. Activation energy of the samples was determined and it is observed
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to be less for urea-doped sample when compared to undoped g-glycine crystal. Acknowledgements The supports extended in the research by Sophisticated Analytical Facility (SAIF), IIT, Chennai, IISc, Bangalore and M.K.University, Madurai are gratefully acknowledged. We thank authorities of Aditanar College of Arts and Science, Tiruchendur and S.T. Hindu College, Nagercoil for the encouragement to carry out the research work. References [1] K. Meera, R. Muralidharan, R. Dhanasekaran, P. Manyum, P. Ramasamy, J. Crystal Growth 263 (2004) 510. [2] S. Aruna, G. Bhagavannarayana, M. Palanisamy, P.C. Thomas, B. Varghese, P. Sagayaraj, J. Crystal Growth 300 (2007) 403. [3] T. Pal, T. Kar, Mater. Chem. Phys. 91 (2005) 343. [4] S.B. Monaco, L.E. Davis, S.P. Velsko, F.T. Wang, D. Eimerl, A.J. Zalkin, J. Crystal Growth 85 (1987) 252. [5] C. Justin Raj, S. Dinakaran, S. Krishnan, B. Milton Boaz, R. Robert, S. Jerome Das, Optics Comm. 281 (2008) 2285. [6] G. Albrecht, R.B. Corey, J. Amer. Chem. Soc. 61 (1939) 1087. [7] E. Ramachandran, K. Baskaran, S. Natarajan, Cryst. Res. Tech. 42 (2007) 73. [8] B. Narayana Moolya, A. Jayarama, M.R. Sureshkumar, S.M. Dharmaprakash, J. Crystal Growth 280 (2005) 581. [9] Y. Iitaka, Acta Cryst. 11 (1958) 225. [10] M. Narayan Bhat, S.M. Dharmaprakash, J. Crystal Growth 236 (2002) 376.
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