Chapter 5 Growth and Characterization of Pure and Doped Organic Nonlinear Optical Crystal: L-Alanine Alaninium Nitrate (LAAN)

Transcription

1 Chapter 5 Growth and Characterization of Pure and Doped Organic Nonlinear Optical Crystal: L-Alanine Alaninium Nitrate (LAAN) 5.1. Introduction The considerable interest has been shown recently in studying the effect of inorganic and organic impurities on the nucleation, growth and physical properties of some NLO single crystals. The impurity may influence the growth, mechanical, electrical, electronic, surface phenomena and structural defects as well as optical quality of the crystals depending upon the nature of host material and the dopant [85, 87]. Therefore selective additives have been applied vastly in crystal growth industry in order to change the appearance shape of crystals and modify their qualities and physical properties of the crystals for the special purposes [13]. The capture of an impurity in a crystal during its growth from solution is a combined effect of various factors such as solubility of host and the impurity phase, character of the mother phase, interaction between the host and the impurity molecules, relative size of impurity and host ions, similarity in the crystallographic structure of the two phases and other crystallization conditions. Also the impurity effect depends on the impurity concentration, super saturation, temperature and PH of the solution [121]. It has been reported that doping NLO crystals with organic impurities [84, 88, 89, 9, 122] and ion impurities [81-83, 88, 91] can alter various physical and chemical properties and doped-nlo crystals may also find applications in optoelectronic devices similar to pure NLO crystals. Considering the above, it can be understood that impurity addition to NLO crystals is expected to make it a more interesting material. Hence, understanding the effect of different kinds of impurities on the physical properties of this material needs several more investigations. Motivated by the above consideration, in the present chapter for the first time, the growth and characterization of pure L-alanine alaninium nitrate (LAAN) (C 3 H 7 NO + 2 C 3 H 8 NO 2 NO - 3 ) crystals and LAAN crystals doped with lanthanum oxide (La 2 O 3 ), sodium chloride (NaCl), urea (CH 4 N 2 O), glycine (C 2 H 5 NO 2 ) and thiourea (CH 4 N 2 S) have been investigated in details. The L-alanine alaninium nitrate (LAAN) belongs to the family of organic nonlinear optical material and grown from its aqueous solution by slow evaporation technique at room temperature. The lanthanum oxide and sodium chloride are well known as ionic substances and are expected to exist as ions in the doped crystal [87-88, 13]. Urea, glycine and thiourea are interesting organic material having large dipole moment and ability to form an extensive network of hydrogen bonds. Also it is believed that the 89

2 incorporation of these impurities expected to occupy the interstitial positions and makes the UV cut-off wavelength of the doped crystal to be less than that of undoped crystal [88-9]. The molecular structures of these selected organic impurities are: Urea Glycine Thiourea We have investigated for the first time the effect of lanthanum oxide, sodium chloride, urea, glycine and thiourea as impurities on the properties of LAAN and results obtained are reported and discussed in this chapter Crystal Growth Synthesis of Pure and Doped LAAN The single crystal of LAAN was synthesized from L-alanine (C 3 H 7 NO 2 ) (99.9 % purity) and nitric acid (HNO 3 ) (72 % purity) taken in the stoichiometric ratio 2:1. The required quantity of L-alanine and nitric acid was estimated according to the following chemical reaction: 2 C H 3 H OH H OH... O +NH 3 + HNO H C 3 3 CH 3 NH2 O +NH 3 O O - H L-alanine + nitric acid L-alanine alaninium nitrate. - NO3 The mechanism of this reaction is given below: 2 HO O CH 3 2 O O - CH 3 NH 2 + NH 3 HNO 3 H + + NO 3 - O - + CH H 3 2 O + H N O NO 3 + NH 3 H 3 C O H O - CH 3 O +NH 3. - NO3 The calculated amounts of L-alanine and nitric acid were dissolved in double distilled water. In order to synthesize the doped LAAN with lanthanum oxide (99.9 % purity), 9

3 sodium chloride (99.9 % purity), urea (99. % purity), glycine (99. % purity) and thiourea (99. % purity), 6 mol% of these additives were added to the solution of LAAN separately. The solutions of pure and doped LAAN were prepared separately and stirred well using a magnetic stirrer for about 2 hours. The solutions were heated at 5 C until the synthesized salts of pure and doped LAAN were obtained Determination of Solubility The size of a crystal depends on the amount of material available in the solution which in turn is decided by the solubility of the material in that solvent. Solubility must be moderate and should have positive temperature gradient in a selected solvent. The solubility of the pure and doped LAAN in double distilled water was studied gravimetrically at different temperatures (3, 4, 5, 6 and 7 C). The solutions of pure and doped LAAN were prepared separately and kept at constant temperature for two hours with constant stirring. The homogeneous solutions were kept in the container for an hour without any disturbance. A 1 ml saturated solution of each sample was pipetted out, dried in oven and weighed to measure the dissolved solute. The same process was repeated for different temperatures range from 3 to 7 C in steps of 1 C. The solubility curves of the pure and doped LAAN crystals are shown in Figure 5.1. It can be seen that the solubility increases with temperature for pure and doped LAAN, thus the double distilled water was used as a solvent throughout the experiment. The solubility of doped crystals was found to be less than that of pure crystal. Concentration (gm/1ml) Lanthanum oxide doped LAAN Temperature ( C) Figure 5.1: The solubility diagram of pure and doped LAAN crystals 91

4 5.2.3 Single Crystal Growth The pure and doped LAAN crystals were grown by dissolving the synthesized salts in appropriate amount in double distilled water and heated at a constant temperature of 5 C with continuous stirring using magnetic stirrer for two hours to form saturated solutions. The saturated solution of each sample was then filtered using filter papers. The filtered solutions were kept in borosil beakers covered with porous thin plastic sheet and allowed to crystallize by slow evaporation of the solvent at room temperature. The colourless crystals of pure LAAN ( 1.7 cm 1.3 cm), lanthanum oxide doped LAAN ( 3. cm 2.2 cm), sodium chloride doped LAAN ( 1.3 cm.8 cm), urea doped LAAN ( 4.6 cm 1.8 cm), glycine doped LAAN ( 2. cm.8 cm) and thiourea doped LAAN ( 1.9 cm.7 cm) were harvested within three to four weeks and are shown in Figure 5.2. Lanthanum oxide doped LAAN Sodium chloride doped LAAN Urea doped LAAN Glycine doped LAAN Thiourea doped LAAN Figure 5.2: The photographs of the as-grown pure and doped LAAN crystals 5.3. Characterization Single Crystal X-Ray Diffraction The single crystal X-ray diffraction analysis of the pure and doped crystals were carried out to identify the structure and to estimate the lattice parameters. The lattice 92

5 parameter values recorded from single crystal XRD analysis for the pure and doped LAAN crystals are given in Table 5.1. This analysis revealed that the pure and doped LAAN crystals belong to monoclinic system with the space group P2 1 which is recognized as non-centro-symmetric, thus satisfying one of the basic and essential material requirements for the SHG activity of the crystals. The obtained crystallographic data of pure LAAN are in good agreement with the reported values [123]. From Table 5.1, it can be seen that, there is no change in the (monoclinic) phase structure of the doped samples; however the slight changes in lattice parameters were observed for the doped samples compared to pure LAAN crystal. The presence of dopants in LAAN crystal may produce lattice strain which leads to change in unit cell parameters of the doped samples. Adsorption of organic additives on the surface of the crystal takes place during the growth, and it is possible that this adsorption of the dopant during the growth may be introduced in the interstitial positions of doped crystal. Hence, the presence of dopants may produce lattice strain which leads to change in unit cell parameters in the doped sample [88-9]. Also, the crystal is subjected to both compressive and tensile strains, respectively around the vacancy and interstitial/substitutional defects due to incorporation of electrically unbalanced ions which leads to change in unit cell parameters in the doped samples [87, 124]. Table 5.1: Unit cell parameters of pure and doped LAAN crystals Crystals a (Å ) b (Å) c (Å) β ( ) Volume (Å 3 crystallite ) size (nm) Lanthanum oxide doped LAAN Sodium chloride doped LAAN Glycine doped LAAN Thiourea doped LAAN Powder X-Ray Diffraction The powder X-ray diffraction analysis was used to confirm the identity of the synthesized salt and to observe the change in diffraction pattern of the doped crystals. The XRD patterns of pure and doped LAAN crystals were recorded in 2θ range from 2 to 8. The powder X-ray diffraction patterns of pure and doped LAAN samples 93

6 are shown in Figure 5.3. The well-defined peaks at specific 2θ values show high crystallinity of the grown crystals. The general observation is that the relative intensities have been reduced and a slight shift in the peak position is observed as a result of doping and indicates the incorporation of additives in to crystal lattice of doped LAAN crystal [87, 89]. These observations could be attributed to strains in the lattice due to doping. Intensity (CPS) θ (degree) Intensity (CPS) 4 Lanthanum oxide doped LAAN θ (degree) Intensity (CPS) 25 Sodium Chloride doped LAAN θ (degree) Intensity (CPS) (12) 2θ (degree) Intensity (CPS) Intensity (CPS) θ (degree) θ (degree) Figure 5.3: The powder X-ray diffraction patterns of pure and doped LAAN crystals 94

7 formula [125]. The crystallite size of pure and doped samples was calculated using Scherrer kλ L = 5.1 W cosθ where k = 1, λ = Å and W is full width at half-maxima of the peaks at the diffracting angle θθ. The calculated crystallite size of the pure and doped LAAN samples is given in Table 5.1. From Table 5.1, it can be seen that, there are changes in the crystallite size after doping due to compressive strain field generated in the sample FTIR Spectral Measurement The FTIR spectral analysis of pure and doped LAAN crystals were carried out in the middle IR region between 4 4 cm -1 to analyze the presence of functional groups in the crystals. Figure 5.4 (a) shows the Fourier transform infrared spectrums of pure LAAN crystal. In the higher energy region there is a broad intense band due to N-H stretch of NH + 3. There is a fine structure in the lower energy region of the band due to hydrogen bonding of NH + 3 with COO - in the crystal lattice for pure LAAN. The observed bands due to the fundamental vibration are in good agreement with the available literature data [ ]. In the overtone region, there is sharp intense peak at cm -1 which is assigned to combinational and asymmetrical bending vibration of NH + 3. The other functional group assignments of the observed peaks are given in Table 5.2. The FTIR spectrums of doped LAAN crystals were recorded and shown in the Figure 5.4 (b to f). All the spectrums shown in the higher energy region, broad intense bands in the range cm -1 are assigned to N-H stretching vibration of NH + 3 groups. The absorption band due to methyl and methane groups appears weakly resolved just below 3 cm -1. The sharp intense peaks which observed in the range cm -1 were assigned to the hydrogen bonding of NH + 3 and COOH group. The peaks which appear between cm -1 are assigned to the asymmetric NH + 3 vibrations, simultaneously the peaks in the range cm -1 are due to asymmetric COO - vibration. The bending modes of CH 3 are resolved in the region cm -1 and cm -1. The peaks at cm -1 are due to C-O stretch and the O-H bend of COOH group observed at cm -1. The presence of NO - 3 is proven by the peaks at cm -1, cm -1 and cm -1. From the IR spectrums, it is clearly that the COO - of pure and doped LAAN are protonated by nitric acid. The comparison of IR spectrums of pure and doped LAAN show slight shift in absorption bands which may be due to presence of doping. 95

8 (a) (b) Wavenumber (cm -1 ) Wavenumber (cm -1 ) (c) (d) Wavenumber (cm -1 ) Wavenumber (cm -1 ) (e) (f) Wavenumber (cm -1 ) Wavenumber (cm -1 ) Figure 5.4: FT-IR spectrums of (a) pure and (b) lanthanum oxide, (c) sodium chloride (d) urea (e) glycine and (f) thiourea doped LAAN crystals 96

9 Table 5.2: FT-IR peak assignments of LAAN crystal FT-IR cm -1 Assignments δ ring C-O in plane deformation NO Asymmetric ring stretching C-H out of plane deformation δ C-N in plane δ C-H Phenolic O δ (O-H) ν (NO 2 ) C-H in plane deformation ν (COO) ν as (COO) CH 2 wagging δ: bending; ν: stretching; as; asymmetric UV-Visible Spectral Measurement The UV-visible spectrums of pure and doped LAAN crystals were recorded in the range of 25 to 8 nm. The absorption and transmittance spectrums of the pure and doped crystals are shown in Figure 5.5 and Figure 5.6 respectively. From Figure 5.5, it can be seen that there is very little absorption at the wavelength of 532 nm, which can improve the second harmonic generation. There is no absorption band between 28 and 8 nm, hence the pure and doped crystals are expected to be transparent to the UV-Visible radiation in between these two wavelengths. From the optical transmittance spectrums (Figure 5.6) of pure and doped LAAN crystals one can see that there is no change in the transmittance window due to doping and all the crystals have a low UV cut-off in the range of nm The percentages of transmittance in doped samples were found to be more when compared to the pure LAAN. The pure and doped LAAN crystals have good transmittance in UV as well as in visible region which is an added advantage for the crystals to be used in optoelectronic applications and it is one of the additional key requirements for having efficient NLO characteristics [7]. The pure and doped crystals have low UV cut-off in the range of nm which is suitable for SHG laser radiation of 164 nm or other application in the blue region. Increase of transparency indicates improvement of quality of crystals with the incorporation of impurities which suppress the inclusions during growth and improve the quality of the crystal with improved transparency [85]. 97

10 Absorbance 4. Lanthanum oxide doped LAAN Wavelength (nm) Figure 5.5: The absorption spectrums of pure and doped LAAN crystals Transmittance (%) Lanthanum oxide doped LAAN Wavelength (nm) Figure 5.6: The transmission spectrums of pure and doped LAAN crystals (αhν) 2 (ev 2 m -2 ) 4x1 1 Lanthanum oxide doped LAAN 3x1 1 2x1 1 1x hν (ev) Figure 5.7: (αhv) 2 vs photon energy hv plot of pure and doped LAAN crystals 98

11 The optical band gaps of the pure and doped crystals were calculated using the equation (3.1). From the transmittance spectrums, a graph is drawn between hνν and (ααhνν) 2 and is displayed in Figure 5.7. The band gap energy of pure and doped crystals is evaluated by extrapolating a straight line in the linear region of the graph at (ααhνν) 2 =. The UV transparency cut-off wavelength and band gap energies for the pure and doped LAAN crystals are tabulated in Table 5.3. It can be seen from the table, that the cut-off wavelength is almost same for pure and doped LAAN. Table: 5.3: The UV transparency cut-off wavelength and band gap energies of the pure and dope LAAN crystals Crystals Cut-off Band gap wavelength (nm) energy (ev) Lanthanum oxide doped LAAN Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) Measurements The SEM studies give information about surface morphology and also can be used to check the presence of imperfections in the grown crystals. The SEM images of the pure and doped LAAN crystals were recorded and are shown in Figure 5.8. The SEM photos exhibit the effectiveness of the impurity in changing the surface morphology of LAAN crystal. Further, analysis of the surface at different sites indicates that the incorporation is non-uniform over the surface, connected with adsorption mechanisms [87, 91]. 99

12 (a) (b) (c) (d) (e) (f) Figure 5.8: The SEM micrographs of (a) pure and (b) lanthanum oxide, (c) sodium chloride (d) urea (e) glycine and (f) thiourea doped LAAN crystals The energy dispersive X-ray (EDX) has become an important tool for characterizing the elements present in the crystal and to determine the chemical composition of these elements. The pure and doped crystals were subjected to EDX to confirm the presence of doped elements in the grown crystals. Incorporation of dopants into the crystalline matrix was observed by EDX spectrums and is shown in Figure 5.9. The weight percentages (wt %) of carbon (C), nitrogen (N), oxygen (O), lanthanum (La), sodium (Na), chlorine (Cl) and sulfur (S) as obtained from EDX analysis for pure and doped crystals are compared and presented in Table 5.4. From the experimental data, the presence of dopants in the doped crystals can be easily identified. It appears that only a small quantity is incorporated into the lattice of the LAAN crystal. 1

13 Lanthanum oxide doped LAAN Figure 5.9: EDX spectrums of pure and doped LAAN crystals 11

14 Table 5.4: EDX quantification data of pure and doped LAAN crystals Lanthanum oxide doped LAAN Sodium chloride doped LAAN Glycine doped LAAN Thiourea doped LAAN Element C N O Total Wt (%) Atomic% Element C N O La Wt (%) Atomic% Element C N O Na Cl Wt (%) Atomic% Element C N O Wt (%) Atomic% Element C N O Wt (%) Atomic% Element C N O S Wt (%) Atomic% Dielectric Measurement Rectangular specimens of pure and doped LAAN crystals were subjected to dielectric studies. The both surfaces of the samples were coated with silver paste to ensure good electrical contacts. The samples were subjected to dielectric studies in the frequency range from 2 Hz to 1 MHz with the temperature range from 3 to 9 C The dielectric constant ε r has been calculated using the equation (2.1). Figure 5.1 shows the variation in dielectric constant with frequency for pure and doped LAAN crystals. It can be observed that as the frequency increases the dielectric constant decreases and for high frequency region it remains almost constant for pure and doped LAAN crystals. The high value of dielectric constant at low frequencies may be due to the contributions from all the four polarizations, electronic, ionic, orientational and space charge polarizations. As the frequency of the applied field increases, a point will be reached where the space charge polarization cannot sustain and comply with variation of the external field and hence the polarization decreases [7]. In accordance with Miller rule, the lower value of dielectric constant at high frequency for a given sample is a suitable parameter for the enhancement of SHG coefficient [88, 128]. From Figure 5.1, it can be seen that the dielectric constant of doped crystals are more 12

15 when compared to pure crystal. The increase in dielectric constant for doped crystals is mainly due to the lattice disorder. Dielectric constant 14 Lanthanum oxide doped LAAN Log f Figure 5.1: The variation of dielectric constant with Log frequency of pure and doped LAAN crystals Figure 5.11 shows the variation in dielectric loss tan δ with frequency for pure and doped crystals. From this figure, it can be seen that the dielectric loss decreases as the frequency increases for all samples. It can also be seen that, the dielectric loss is higher at lower frequencies for doping crystals when compared to the pure crystal. The characteristic of low dielectric loss with high frequency for a given sample shows that the sample possesses good optical quality with lesser defects and this parameter is of vital importance for nonlinear optical materials in their application [129]. The nature of decrease of dielectric constant and dielectric loss with frequency suggests that the grown crystals 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. When urea, glycine and thiourea are added as the dopant, it is possible that doping may occupy the interstitial positions of lattice of LAAN crystal and the presence of defects (dopants) may be responsible for increase in the space charge polarization and hence there is an enhancement of dielectric constant and loss values in case of doped samples in the low frequency region [13]. 13

16 Dielectric loss Lanthanum oxide doped LAAN Log f Figure 5.11: The variation of dielectric loss with Log frequency of pure and doped LAAN crystals The ac conductivity σσ aaaa is calculated by substituting the value of dielectric constant ε r and dielectric loss ttttttδδ in the relation (2.2). Figure 5.12 shows the response of ac conductivity with frequency in the range from 2 Hz to 1 MHz for pure and doped LAAN crystals. From the Figure 5.12, it can be seen that ac conductivity was found to increase after doping and the observed ac conductivity was found to be more at higher frequencies due to a reduction in the space charge polarization. The ac conductivity of the crystal was found to increase after doping owing to the fact that more defects are created after doping. Figure 5.13 and 5.14 show the variation of dielectric constant and dielectric loss with temperature at a constant frequency of 1 KHz for pure and doped LAAN crystals. The variation of the dielectric parameters with temperature is generally attributed to the crystal expansion, the electronic, space charge, 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 [13]. 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. Thermal energy will proportionally activate more surface charges to induce the surface polarization apart to bound charge polarization. Hence net 14

17 polarization gets enhanced with respect to increase in thermal energy. Therefore dielectric constant increases and correspondingly dielectric loss also varies [131]. σ ac 1 6 ( Ω.m) Lanthanum oxide doped LAAN Log f Figure 5.12: The variation of ac electrical conductivity versus frequency of pure and doped LAAN crystals Dielectric constant Lanthanum oxide doped LAAN Temperature ( C) Figure 5.13: The dielectric constants versus temperature of pure and doped LAAN crystals 15

18 Dielectric loss Lanthanum oxide doped LAAN Temperature ( C) Figure 5.14: The dielectric loss versus temperature of pure and doped LAAN crystals Figure 5.15 shows the behaviour of ac conductivity with temperature in the range of 3 C to 9 C for pure and doped LAAN crystals. From Figure 5.15, it can be seen that the ac conductivity for all the samples increase with increase in temperature. The ac conductivity values for pure and doped crystals measured at room temperature with fixed frequency at 1 khz are given in Table 5.5. The electrical conduction in dielectrics is mainly a defect controlled process in the low temperature region and the presence of impurities and vacancies mainly determine this region. The defect concentration will increase exponentially with temperature and consequently the electrical conduction also increases. The measurements show that the doped LAAN crystals have larger conductivity than pure LAAN crystals. Activation energy is the energy required for charge carriers to take part in the conduction process of the samples. The activation energy for the grown crystal was calculated using the equation; - Ea Ea σ = σ Lnσ = Lnσ o exp o 5.2 K BT K BT where E a is the activation energy, K B.is the Boltzmann constant, T is the absolute temperature and σ o is the constant depending on the material. Figure 5.16 shows the plot between Lnσ ac and 1 3 /T for pure and doped LAAN crystals. The ac activation energies were estimated using the slopes of the corresponding lines and are given in 16

19 Table 5.5. From Table 5.5, it can be seen that the activation energy was found to decreases after doping. The decrease in activation energy may be due to the presence of dopants in the doped samples. When LAAN crystals were doped with the impurities, the lattice defects increases and this enhances the conductivity and hence there is decrease in activation energy [79, 13]. The low activation energies observed suggests that oxygen vacancies may be responsible for conduction in the temperature region considered in the present study [8]. σ ac 1 6 ( Ω.m) Lanthanum oxide doped LAAN Temperature ( C) 8 9 Figure 5.15: The ac electrical conductivity versus temperature of pure and doped LAAN crystals Lnσ ac Lanthanum oxide doped LAAN /T (K -1 ) Figure 5.16: The variation of Ln σ ac with 1 T (K-1 ) of pure and doped LAAN crystals 17

20 Table 5.5: The ac conductivity and activation energy values of the pure and doped LAAN crystals at room temperature Crystals AC conductivity (Ω.m) -1 Activation energy (ev) Lanthanum oxide doped LAAN DC Conductivity Measurement The dc conductivity measurements of pure and doped LAAN crystals were carried out at different temperatures ranging from 27 to 11 C. The dc conductivity σ dc of the grown crystal was calculated using the relation; t σ dc = 5.3 RA where, R is the measured resistance, tt is the thickness of the crystal and A is the area of the face of the crystal in contact with the electrode. The I-V characteristic curves of pure and doped crystals at room temperature are shown in Figure 5.17 and from which the conductivity values were calculated. The dc conductivity values at room temperature are given in Table 5.6. The dc conductivity was found to increase after doping. The existence of π electrons in the organic impurity will contribute to the conductanse of LAAN crystal after doping. As the organic impurities mainly occupy the interstitial positions and the impurity concentrations considered in the present study are small, the impurity molecules can be assumed to be added in the LAAN lattice in the same amount (ratio) as estimated. Hence, the total dc conductivity increases due to the addition of defects in the form of impurity molecules [84]. Figure 5.18 shows the plot of dc conductivity with temperature for pure and doped crystals. From Figure 5.18, it can be seen that, the dc conductivity was found to increase with increase in temperature for all samples before and after doping. The defect concentration will increase exponentially with temperature and consequently the electrical conduction also increases [8, 84]. The conductivity graph (Figure 5.18) exhibits the intrinsic and extrinsic regions. The conductivity at high temperature is intrinsic, which is due to the thermally created vacancies and defects created in 18

21 crystalline lattice. The extrinsic region at low temperatures is a structure-sensitive region i.e., electrical conductivity is controlled by impurities. Hence, the addition of impurities further increases the electrical conductivity in the extrinsic region [132]. Figure 5.19 shows the plots between Lnσ dc and 1 3 /T for pure and doped samples. The dc activation energies were estimated using the slopes of the corresponding lines and is given in Table 5.6. From Table 5.6, it can be seen that the activation energy was found to decreases after doping. Current (Α) 3.x x1-8 Lanthanum oxide doped LAAN 2.x x1-8 1.x1-8 5.x Voltage (V) Figure 5.17: The I-V graph of pure and doped LAAN crystals σ dc (1-8 mho/m) Lanthanum oxide doped LAAN Temperature( C) Figure 5.18: The dc electrical conductivity versus temperature for pure and doped LAAN crystals 19

22 Lnσ dc Lanthanum oxide doped LAAN /T(K -1 ) Figure 5.19: The variation of Lnσ dc with 1 T (K-1 ) for pure and doped LAAN crystals Table 5.6: The dc conductivity and activation energy values of the pure and doped LAAN crystals at room temperature Crystals DC conductivity (Ω.m) -1 Activation energy (ev) Lanthanum oxide doped LAAN Refractive Index (RI) Measurement The refractive index of the pure and doped crystals is measured using Brewster's angle method. The source of monochromatic light used was nm He-Ne laser. The polished surface of the grown crystal was used to reflect the He-Ne laser beam for Brewster's angle measurement. The reflected beam was scanned by means of a laser beam detector. The observations were repeated with different angles of incidence. The refractive index (RI) was calculated using the relation; RI = tanθ P 5.4 where θ p is the polarizing angle. The polarizing angle θ p and the results are tabulated in Table

23 Table 5.7: Refractive index values of pure and doped LAAN crystals Crystals Brewster s angle Refractive Index Lanthanum oxide doped LAAN From the table 5.7, it can be seen that, the RI of the crystals was found to decrease slightly after doping Microhardness Measurement The microhardness of a material is a measure of the resistance it offers to local deformation. The polished surface of pure and doped crystals was subjected to static indentation tests at room temperature using a Vicker s microhardness tester attached to a large incident light microscope to observe the indentation mark on the material surface and measure the diagonal length (in μm) of the indentation mark. The loads ranging from 1 to 1 gm were used for making indentations, keeping time of indentation constant at 1 s. The microhardness value was calculated using the equation; P d 2 H V = kg/mm where, H V is the Vicker s hardness number, P is the applied load in kg, d is the average diagonal length of the indentation impression in mm and is a constant of a geometrical factor for the diamond pyramid. The variation of Vicker s hardness number H V with applied load of pure and doped LAAN crystals is shown in Figure 5.2. From Figure 5.2, it can be seen that the H V of pure and doped LAAN crystals increases with load and this is due to the elastic nature of the crystal lattice, which will produce the restoring force against the applied load and try to restore the lattice to its original position. In the low load regime, the indenter penetrates only the upper surface layer of the crystal and depending on the strain distribution of the upper surface layer there is increase of hardness value in low load region. With the increase of load, the depth of penetration of indenter increases and both the effects of inner 111

24 layer and surface layer contribute to the hardness, which vary nonlinearly with load. The effect of inner layers becomes more and more prominent at high load and ultimately no change is observed in hardness with the variation of load [5]. Also, H V was found to increase after doping for all loads. The possible explanation for this behaviour is may be due to defects created in the doped samples which act as obstacle to dislocation motion thus increasing the hardness of the crystal [12]. Also the increase in the hardness value of doped samples can be attributed to the incorporation of impurity in the lattice of crystal. The addition of impurities to crystalline sample most probably enhances the strength of bonding with the host material and hence hardness number increases [13] H V (kg/mm 2 ) Lanthanum oxide doped LAAN Load P (1-3 kg) Figure 5.2: The variation of Vicker s hardness number with load of pure and doped crystals The Meyer s index number was calculated from Meyer s law, which relates the load and indentation diagonal length, n P = kd 5.6 Log P = Log k + n Log d 5.7 where, k is the constant for the given material and n is Meyer s index or work hardening index. Figure 5.21 shows a graph of Log P versus Log d of pure and doped LAAN crystals which gives a straight line; the slope of this straight line gives the value of n. The calculated values of n for pure and doped LAAN crystals are tabulated in Table 5.8. According to Onitsch [133], H V should increase with P if n>2 112

25 and decrease if n<2 and n should lie between 1 and 1.6 for harder materials and above 1.6 for softer materials. Hence the grown crystals of this work belong to the category of soft materials. Log P 2.4 Lanthanum oxide doped LAAN Log d Figure 5.21: The plot of Log P versus Log d of pure and doped LAAN crystals Table 5.8: The Meyer s index values of the pure and doped LAAN crystals Crystals Meyer s index 2.4 Lanthanum oxide doped LAAN Thermal Measurements The differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) of pure and doped crystals were carried out simultaneously between 3-4 C and the obtained TGA/DTA trace of the crystals are shown in Figure The TGA curve of pure crystal shows a sharp weight loss at around C which is attributed to the volatilization of crystal. Below the onset of volatilization no loss of weight was observed and hence it is clear that no lattice entrapped water inside the crystal. In the case of lanthanum oxide, urea and glycine doped crystals, the major weight loss start 113

26 at C, C and C respectively which is higher than the pure system. From the DTA curve of pure crystal, it was observed that an endothermic peak at C is corresponding to the melting point of the sample. In the case of lanthanum oxide, urea and glycine doped crystals, the materials was melted at C, C and 15.9 C respectively, which show that the melting point and thermal stability of the material was increased for the doped samples. The pure and doped samples can be used for NLO applications up to its melting point [6, 134]. The sharp endothermic peaks which occur just above the melting of pure and doped LAAN crystals are due to oxidation decomposition of the compound. These endothermic closely match with the major weight loss in TG analysis. Weight percent (%) C C C C Temperature ( o C) Heat flow (W/gm) -9-1 Weight percent (%) Lanthanum oxide doped LAAN C C Temperature ( o C) C C Heat flow (W/gm) 114

27 Weight percent (%) C C C C Heat flow (W/gm) C Temperature ( o C) C Weight percent (%) C C C Temperature ( o C) Heat flow (W/gm) Figure 5.22: TGA/DTA curve of pure and doped LAAN crystals Powder SHG Measurement The SHG conversion efficiency of pure and doped LAAN crystals were estimated using modified setup of Kurtz and Perry at the Indian Institute of Science (IISc), Bangalore [14]. A Q-switched Nd:YAG laser beam of wavelength 164 nm was used with an input power of 2.15 mj and pulses width 1 ns with a repetition rate of 1 Hz. The signal amplitude in mv on the oscilloscope indicates the SHG efficiency of the grown crystals. The SHG efficiency of pure and doped crystals was estimated with respect to standard KDP and is given in Table 5.9. From Table 5.9, it can be seen that 115

28 the SHG efficiency of pure LAAN crystal increases after doping with all impurities. The second order optical properties of organic crystals are very sensitive to the number of crystal imperfections and defects. Due to the presence of dopant in the crystal lattice, there is an increase in polarizability of the molecule, which tends to increase the SHG efficiency [9]. Table 5.9: SHG efficiency of the pure and doped LAAN crystals Crystals SHG signal Efficiency with respect (mv) to KDP (11.5 mv) Lanthanum oxide doped LAAN Conclusions The single crystals of pure LAAN and doped with lanthanum oxide, sodium chloride, urea, glycine and thiourea were grown by slow evaporation technique at room temperature. Single crystal X-ray diffraction analysis was carried out to calculate the lattice parameters of the grown crystals. The FTIR study was used to evaluate the functional groups present in the doped LAAN crystals. The UV-visible spectrum reveals that there is no change in transmission window for doped LAAN crystals as compared to that of pure crystal. The good transmission property of the pure and doped LAAN crystal in the entire visible region ensures its suitability for SHG applications. The SEM photographs exhibit the effectiveness of the impurity in changing the surface morphology of pure LAAN crystal. The incorporation of dopants into the crystalline matrix was observed by EDX. The dielectric constant was found to be less in pure LAAN crystal when compared to doped crystals. The low value of dielectric constant and dielectric loss at higher frequencies suggests that the crystals possess enhanced optical quality which has significant role in the NLO applications. The ac and dc conductivity were found to increase after doping due to the induced defects in crystal lattice. The microhardness study of the doped crystals indicates relatively higher hardness values than the pure LAAN crystals. The melting points of the doped crystals were found to be high when compared to pure crystal. The SHG conversion efficiency measurement shows that the doping enhances the SHG efficiency of pure LAAN crystal. 116