Growth and Charactersation of L-Serine Doped Pottassium Dihydrogen Phosphate Crystal

Growth and Charactersation of L-Serine Doped Pottassium Dihydrogen Phosphate Crystal

K. Manimekalai a*

Department of Physics Govt.Arts College Thiruvannamalai, India

R. Rajasekaran a Department Of Physics

Aruna Vidya Arts & Science College,Kannakurukai Thiruvannamalai,India

Abstract Potassium dihydrogen phosphate is a material rich in NLO property with wide range of industrial applications. Certain Amino acids are also rich in NLO properties so they are used as dopants.In the present work L- Serine a non essential amino acid is added in the ratio 1:1 molar percentage to KDP. Single crystals were grown by slow evaporation method. The grown crystals were characterized by powder X-ray diffraction, Single crystal X-Ray diffraction, UV vis-spectroscopy, Fourier transform infrared spectroscopy (FTIR), SHG efficiency and dielectric studies, The single crystal X-ray diffraction studies reveal the structure and space group of the crystal. The powder X-ray diffraction study shows that the crystalline perfection of grown crystal is good. Fourier transform infrared studies confirm the functional groups of the crystals. It is seen from UVvis studies that the optical transparency is found to increase much by adding chlorides. SHG efficiency of KDP is found to be slightly increasing with doping of L-Serine.The electrical properties of the crystal have been studied using dielectric constant studies. All the results are compared with the results of pure KDP.

Keywords: KDP, XRD, UV-Vis, SHG, FTIR

1. INTRODUCTION

   Nonlinear optic (NLO) crystals are used for harmonic generation, including frequency doubling (SHG), tripling (3HG), frequency mixing; OPO(Optical Parametric Oscillator) and OPA(Optical Parametric Amplifier)[1].Combining harmonic generation and frequency mixing, we can generate the 4th, 5th, or even 15th harmonics from the original laser output.

   Potassium Dihydrogen Phosphate (KDP) the most widely- used commercial NLO materials. They possess many excellent properties like good UV transmission, High optical damage threshold, High birefringence and High non linear coefficients. They are commonly used for doubling, tripling and quadrupling of Nd:YAG laser at the room temperature. In addition, they are also excellent electro-optic crystals with high electro-optic coefficients, widely used as electro-optical modulators, Q-switches, Shutter for high speed photography, High power laser frequency conversion materials etc. The aim is to develop materials presenting large non linearity and satisfying at the same time all the technologies requirements for applications such as wide transparency range, fast response and high damage threshold. But in addition to the processability, adaptability and interfacing with other materials improvements in non linear effects in devices, led the way to study of new non linear effects and the introduction of new concepts.. Potassium dihydrogen

   phosphate (KDP) is an excellent inorganic nonlinear optical (NLO) material and has considerable interest amongst several research workers because of its wide frequency, high efficiency of frequency conversion, high damage threshold against high power laser. With the aim of improving the SHG efficiency of KDP, researchers have attempted to modify KDP crystals either by doping different type of impurities or by changing the growth conditions[2-9]. In the present study an effort has been made in growing KDP doped with L- Serine by slow evaporation growth technique. The grown crystals are subjected to various spectral analysis like FTIR, UV-Visible, single crystal XRD, powder XRD and Dielectric studies

2. EXPERIMENTAL PROCEDURE

   1. Synthesis and Crystal Growth

      Pure KDP AR grade E-Merck and L-serine doped crystals were grown by slow evaporation growth technique at room temperature. The saturated solution is prepared by dissolving the solute of KDP in 30g per 100ml in Millipore water. The doping of L-Serine was carried out by adding 0.1 weight percentage powder form of L-Serine into 1 molar 100 ml solution of KDP in millipore water. The mixtures were well stirred for 8 hrs for homogenization .Then it was double times filtered with whatt man filter paper and poured into Petri dishes and covered with perforated polyethylene. The prepared solution was allowed to dry at room temperature and the crystals were obtained by slow evaporation method. In the period of 05-12 days, the crystal had formed as shown in Fig. 1.Then the grown crystals has been subjected various spectral studies to analyze its characteristics .Grown crystals were found to be colour-less and transparent. Figure 1 shows a photograph of 0.1 wt. L-serine doped KDP.

      Fig(1) As grown crystals of L-serine doped KDP

   2. Characterization

      The grown L-serine doped KDP crystals were subjected to various characterizations .Single crystal X-ray diffraction studies were carried out using single crystal diffractometer ENRAF NONIUS Cad4 and its lattice parameter volume structure and space group is analysed in given in table. Powder X-ray diffraction studies were carried out using powder X-ray diffraction instrument D8 advanced BRUKER Spectrometer using CuK radiation source and its wave length (=1.54), data collected from the 2 range from 10°to 90° in steps of 0.020 and count time 0.2S.The obtained results are in Fig.2(a) .Identification of functional groups were carried out by FTIR analysis using JASCO 4100 shown in Fig 3.UV-Visible spectral study is carried out using SHIMADZU 2600 in the range 200-1200nm as shown in Fig.4(a) and the results were compared with that of pure KDP and a comparative studies are made between the L- serine doped KDP Crystals. The NLO property of the doped crystal is evaluated by the Krutz and Perry (1968) powder technique using a Q-switched, mode locked Nd : YAG laser. Dielectric property of the crystal is carried out for various temperature using HIOKI MODEL 3532_50 LCR High Tester.

3. RESULTS AND DISCUSSION

KDP crystal is a promising NLO material with high transparency and frequency doubling property when doped with L-serine shows some changes in its Characteristics when subjected to Single crystal X-ray diffractiom , powder X-Ray diffraction , Fourier transform Infra red Spectroscopy ,UV Spectroscopy, SHG measurements an Dielectric measurements.

1. SINGLE CRYSTAL XRD STUDIES:

   Single crystal X-ray diffraction analysis was carried out using ENRAF NONIUS Cad4 diffractometer to identify the lattice parameters. The single crystal X-ray diffraction studies confirm the tetragonal structure with the space group of I- 42d .The lattice parameters of L-Serine doped are; a=b=7.451A , c=12.346A , with volume V=387.35A 3. The crystal parameters and cell volume were found to be well in agreement with reported values [10] as shown in table(1).From the grown doped crystal lattice parameters and space group it is clear that the basic structural property and space group of KDP is not altered by the dopant[11].

   Table-1 Lattice parameters for pure KDP and L-serine doped KDP

   Compoud	Crystal system	Space group	Unit cell parameters
   KDP	Tetragonal	I-42d	a=b=7.455Ã…, c=6.975 Ã…. ===90o
   KMS5	Tetragonal	I-42d	a=b=7.4515Ã…,c=6.99 ===90o

2. POWDER XRD STUDIES:

   The L-serine doped KDP crystals when subjected to X-ray diffraction shows pattern as in fi 2 .While comparing with the powder xrd pattern of pure KDP it is found that 2theta values shifted and this suggests that the structure of KDP is slightly disturbed by of L-serine which tries to change the transparency.

   7000

   6000

   5000

   intensity(count)

   4000

   3000

   2000

   1000

   0

   0 10 20 30 40 50 60 70 80

   2

   Fig:2 Powder XRD Pattern of L-serine doped KDP

3. FOURIER-TRANSFORM INFRARED SPECTROSCOPY STUDIES:

   FTIR is a technique which is used to obtain an infrared spectrum of absorption, emission or photoconductivity of a solid, liquid or gas. It collects high spectral resolution data over a wide spectral range. This analysis has been carried out by recording the spectrum in the range 4000 400 cm-1 using KBr pellet method. The spectrum of L-Serine doped KDP is shown in fig 3.The IR spectra in1450 -600 is called the fingerprint region so it is difficult to assign all absorption bands this region. The already reported pure KDP crystal shows O-H stretching at frequencies 3615 cm-1, 3320 cm-1 and 3155cm-1. P-O-H . Stretching of H2PO4 arises at 2914

   cm-1, at 1136 it shows P=O stretching and at 531 shows HO- PO-OH stretching in the spectrum. Table -2 shows the functional group assignments and their frequencies [12].

   100

   95

   90

   %T 85

   80

   75

   70

   65

   60

   4000 3500 3000 2500 2000 1500 1000 500

   -2

   wavelength (cm )

   Frequency cm-1	Fuctional group assignments
   3437.49	Asymmetric stretching ofNH3+
   2924.52	P-O-H stretching
   2460.72	C=H stretching
   1637.27	P-O-H bending
   1306.54	N-H bending of dopants
   1096.37	P=O stretching
   904.45	P=O stretching
   536.11	c-o bending
   457.04	HO-P-OH bending

   Fig.3. FTIR pattern for L-Serine doped KDP Table:2 Functional groups of L-serine doped KDP

   Fig.4(a) UV-Spectrum of L-serine doped KDP

   55

   Fig:4(b) plot of (h)1/2 versus photon energy h

4. UV- VISIBLE SPECTRAL ANALYSIS:

   The optical transmission spectra has been recorded by using SHIMADZU -2600 spectrometer in the range of 200-1200 nm as shown in fig 4.The crystal is highly transparent in the entire visible region, whereas it has a UV cut off at 256. The transmission is uniformly high (73%) for light in the visible region of electromagnetic spectrum, which is useful for nonlinear device application. The resultant spectrum is shown in Fig. 4(a)

   50

   45

   h

   40

   35

   30

   25

   20

   15

   1 2 3 4 5 6 7

   E=h

   Fig:4(c) plot of (h)1/2 versus h showing band gap

   The measured transmittance (T) was used to calculate the absorption coefficient (a) using the formula:

   where t is the thickness of the sample[13].

   As in indirect band gap semiconductor, the crystal under study has an absorption coefficient () obeying the following relation for high photon energies (h).

   (h) = A (Eg h) 1/2

   Where, A is a constant, Eg the optical band gap, h the Plancks constant and

   the frequency of the incident photons. Optical band gap was calculated from the UV-Visible data. The plot between energy (h) and (h)1/2 is made as shown in Figure (4.a) (Where is the absorption coefficient) and the optical band gap energy is found to be 3.4 eV by extrapolating the slope region (where it cuts the X-axis) which is shown in Fig(4.c).The internal efficiency of the device also depends upon the absorption coefficient. Hence by modifying the absorption coefficient and tuning the band gap of the

   material, one can achieve the desired material which is suitable for fabricating various layers of the optoelectronic devices as per our requirements.[14].

   Extinction coefficient (K) can be obtained from the following equation:

   The transmittance (T) is given by

   Reflectance (R) in terms of absorption coefficient can be obtained from the above equation. Hence,

   Reflectance (R) in terms of absorption coefficient can be obtained from the above equation. Hence,

   Reflectance (R) in terms of absorption coefficient can be obtained from the above equation. Hence,

   Refractive index (n) can be determined from reflectance data using the following equation

   +1)

   And it found to be 1.26 at wavelength =1100nm.

5. SIMPLE HARMONIC STUDIES:

   The pure and doped KDP crystals were made into fine powders of the size of 10 µm. The micro particles were exposed to 1064 nm laser beam from a pulsed Nd: YAG laser to test the second harmonic generation efficiency. An input pulse of 1.9 mJ/pulse was supplied. Signal amplitude in millivolts on the oscilloscope indicates the efficiency of the sample. The pure KDP crystal gave an output 73 mV whereas the L-Serine doped KDP crystal showed an increase in the SHG efficiency. The L-Serine doped KDP crystal gave an output of 76mV. Thus, the SHG efficiencies of the doped crystals are 1.04 times greater than the standard KDP crystals respectively[15].

6. DIELECTRIC STUDY

The dielectric analysis is an important characteristic that can be used to fetch knowledge based on the electrical properties of a material medium as a function of temperature and frequency. The dielectric studies were measured using HIOKI MODEL 3532_50 LCR High Tester .Based on this analysis, the capability of storing electric charges by the material and capability of transferring the electric charge can be assessed.

Dielectric properties are correlated with electro optic property of the crystals, particularly when they are non conducting materials [16]. Microelectronics industry needs low dielectric constant (r) materials as an interlayer dielectric[17]

The dielectric constant is calculated using the formula

=

where C is capacitance (F), t the thickness (m), A the area(m2), and o the absolute permittivity in the free space having a value of 8.854 x 1012 Fm1.

res5 (a), 5(b) show the variation of dielectric constant and dielectric loss with respect to frequency for all temperatures of L-Serine doped KDP crystals. Compared to the pure KDP, dielectric constant (r ) value is found to be low for doped KDP[18].

From Figures 5(a) and 5(b), it is clear that dielectric loss is high at low frequency and decreases with high frequencies. The low dielectric loss at high frequency reveals the high optical quality of the crystal with lesser defects, which is a desirable property of NLO applications [19, 20]

dielectric constant and dielectric loss decrease with the increasing frequency. This may be due to the contributions of all the four polarizations such as electronic, ionic, orientation, and space charge, which are predominant in the lower frequency region [21]. The larger value of dielectric constant and dielectric loss at low frequency arises due to the presence of space charge polarization near the grain boundary interfaces, which depends on the purity and perfection of the sample[22].

450

400

350

Dielectric const

300

250

200

150

100

32C

90C

120C

160C

1 2 3 4 5 6 7

Log f

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    1.60

    1.58

    1.56

    1.54

    Dielectric loss

    1.52

    1.50

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    32

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    ACKNOWLEDGMENT

    The Authors are thankful to Dr.N.P.Rajesh (SSN College of Engineering, Kalavakkam,Chennai) for providing Lab facilities for the crystal growth.

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