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Investigation on the influence of foreign metal ions in crystal growth and characterization of l -Alaninium Maleate (LAM) single crystals
Accepted Manuscript Investigation on the influence of foreign metal ions in crystal growth and char‐ acterization of L -Alaninium Maleate (LAM) single crystals L. Ruby Nirmala, J. Thomas Joseph Prakash PII: DOI: Reference:
S1386-1425(13)00714-2 http://dx.doi.org/10.1016/j.saa.2013.06.100 SAA 10721
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐ lecular Spectroscopy
Received Date: Revised Date: Accepted Date:
20 March 2013 24 June 2013 27 June 2013
Please cite this article as: L. Ruby Nirmala, J. Thomas Joseph Prakash, Investigation on the influence of foreign metal ions in crystal growth and characterization of L -Alaninium Maleate (LAM) single crystals, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.06.100
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on the influence of foreign metal ions in crystal growth and characterization of L -Alaninium Maleate (LAM) single crystals L.Ruby Nirmala, J.Thomas Joseph Prakash* PG and Research Dept. Of Physics, Holy Cross College (Autonomous), (Affiliated to Bharathidasan University) Trichy 620 002, Tamil Nadu, India *PG and Research Dept. of Physics, H.H. The Rajah’s College, (Affiliated to Bharathidasan University) Pudukkottai-1. Tamilnadu, India Abstract A Nonlinear Optical, good quality, single crystals of doped and undoped L-Alaninium Maleate (LAM) were grown by slow evaporation solution growth technique at room temperature. The lattice parameters were analyzed by single crystal X-ray diffraction technique. The identification of Cadmium ion in the doped crystals was done using the EDAX spectrum. The presence of functional group of the dopant with LAM molecule was studied using FTIR spectra. The results of UV-Vis study is used to compare the transparencies of the doped and undoped LAM crystals. The optical band gap energy of the grown crystal was also calculated. The relative second harmonic generation (SHG) efficiency measurement with KDP reference is used to find the incorporation of metal to L-Alaninium Maleate crystals and the parent material. Also the thermal stability of the grown crystals was studied by TGA/DTA spectrum. The mechanical stability of the grown crystals was confirmed through Vickers micro hardness study. By parallel plate capacitor technique, the dielectric response was studied over a wide range of frequencies at different temperatures. The various studies showed the incorporation of the impurity Cd2+ into LAM crystals and the investigations indicated that the impurity played an important role in the changes of the spectral and structural properties of LAM crystals. Key words: NLO material; Growth from solution; Second-harmonic Generation; metal doped LAM. * Corresponding author E-mail: [email protected] Tel: +91 09842470521
1. Introduction NLO materials find
extensive opto electronic applications such as
optical
communication, signal processing, frequency conversion, optical data storage, optical switches etc. [1-3]. These applications depend on the properties such as transparency, refractive index, dielectric constant, thermal, mechanical, photochemical and chemical stability [4]. Among NLO materials, organic NLO materials are generally believed to be more versatile than their inorganic counterparts due to their more favorable nonlinear response [5]. Inorganic materials are also used in these applications due to their high melting point, high mechanical strength and high degree of chemical inertness but their optical nonlinearity is poor. The organic compounds with electron rich (donor) and deficient (acceptor) substituent provide the asymmetric charge distribution in the
– electron system and show large nonlinear optical responses. The family of materials of
an amino acid is extensively researched for NLO activity due to the fact that all the amino acids except glycine contain chiral carbon atom and crystallize in noncentro symmetric space groups which is an essential character of NLO activity [6, 7]. The L-Alanine molecule exists in the cationic form (C3H8NO2+) with a positively charged amino group and is an efficient organic NLO material. This was combined with the maleic acid molecule which is in the mono-ionized state (C4H3NO4-) in order to produce the material L-Alaninium Maleate to challenge the NLO materials already existing. The structure of L-alaninium maleate crystal was already solved by Alagar et.al. [8]. The growth and characterization of L-alaninium maleate crystals were reported by many authors earlier [9 - 15]. The Cu2+, Mg2+ metal ions doped on LAM crystal [16] and the rare earth elements like La3+, Nd3+ doped on LAM crystal [17] were also been reported that the presence of dopants improved the nonlinear optical (NLO) properties of the parent material. Motivated by these considerations an attempt is made to grow another optically transparent material to increase in SHG efficiency, such as a transition metal ion (Cd2+) doped LAM single crystals by the slow evaporation method. The characteristic modification by doping foreign metal ion (Cd2+) on the LAM crystal was studied by different characteristic techniques and the report of these investigations was discussed in this paper. 2. Experimental procedure Material synthesis and crystal growth LAM was synthesized using high purity L-alanine and AR grade Maleic acid from Merck in the stoichiometric ratio1:1with double distilled water. The reactants were thoroughly dissolved
in water and stirred well for about 2 hours to yield a homogeneous solution using a temperature controlled magnetic stirrer. The LAM mixture was synthesized according to the following reactions. C3H7NO2 + C4H4O4
C3H8NO2+ . C4H3O4-
The prepared mixture was dried and a saturated solution was prepared using the synthesized salt. The solution was slightly heated upto an optimum temperature of 55 °C and it was filtered twice to remove the suspended impurities. Then the saturated solution was taken in a crystallizing vessel with a perforated lid in order to control the evaporation rate and kept in undisturbed conditions. After three weeks, transparent single crystals of LAM were harvested from the mother solution. The same procedure was applied to grow metal Cd2+ doped crystals by adding 1mol % of Cadmium Chloride to the saturated LAM solution. The incorporation of dopant into the parent solution has promoted the growth rate and improved the quality of the crystals. Successive recrystallisation was done for purification of the grown crystals and the crystals were found to be transparent and free from defects. The photographs of the ‘as grown’ undoped and Cd2+ doped LAM crystals are shown in Fig. S1(see supplementary material) and Fig. S2 (see supplementary material) respectively. 3. Characterization studies, Results and discussion 3.1. Single crystal X- ray diffraction analysis The undoped and Cd2+ doped LAM crystals were subjected to single crystal X-ray diffraction studies using Enrafnonius CAD4 X-ray diffractometer to determine the unit cell parameters. The obtained lattice parameter values of the undoped and doped crystals were tabulated in Table 1 and the values are well matched with the reported literature [8-10]. It was seen that the grown crystals were crystallized in orthorhombic with the space group P212121, a well known noncentro symmetric space group, thus satisfying the requirements for second order NLO activity.
3.2. EDAX analysis Energy dispersive X-ray analysis is a technique used to identify the elemental composition of a sample. The observed EDS spectrum of Cd2+ doped LAM crystal having the peaks attributed to all the elements at different energies was depicted in Fig. S3 (see supplementary material) which confirmed the incorporation of impurity. All the prominent peaks corresponding to different elements in the sample were seen in the spectrum. 3.3. FTIR spectrum analysis Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify mainly, the functional groups present in organic materials. FTIR analysis provides information about the chemical bonds and molecular structure of a material. In order to qualitatively analyze the presence of functional groups in the grown crystals, the
FT-IR
spectrum were recorded in the range 400 cm-1 to 4000 cm-1 using a KBr pellet on Perkin Elmer RXI FTIR spectrometer and the recorded spectra were shown as Figs. 1 and 2. It was observed that a strong absorption occurred at 3209 and 2934 cm-1 corresponding to the stretching bonds of the NH3+ ion of the amino acid. The strong carbonyl absorption at 1723 cm-1 confirmed the asymmetric stretching of COO- group of the compound.
The absorption bands at about
1376 cm-1 and at 1260 cm-1 may be assigned to the C-H deformation in CH3. A strong bond arising from C–COO- vibration is observed at 1215 cm-1 [10]. The absorption peaks at about 759 and at 582cm-1 may be assigned to the rocking of CH2 group and wagging of COO- group respectively. The characteristic vibrations establishing the identity of all the functional groups present in the compounds were presented in Table 2. The presence of additional peaks in the lower frequency region around 409 cm-1 and 902 cm-1 in the doped spectra may be due to the presence of Cadmium in the coordination sphere. Although the spectrum of Cd2+ doped LAM provides similar features as that of undoped LAM, there is a slight shifting observed for all the peaks suggesting a wide range of interactions for the functional groups. It was also observed that there was broadening or narrowing of some absorption peaks in the FTIR spectrum of doped LAM as that of undoped LAM and this may be due to the incorporation of Cd2+ in the lattice of LAM.
3.4. UV-Visible spectrum analysis The optical transmittance spectrum of undoped and Cd2+ doped LAM samples were recorded using a Perkin-Elmer Lambda 35 spectrophotometer in the range of 200 - 1100 nm. The recorded spectra of the grown crystals were shown as Fig. 3. From the spectrum it was observed that both the crystals showed very high transmission at entire IR and visible regions. This showed the optical transparency of the grown materials. For the undoped LAM the UV cutoff wavelength is found at 340 nm and the doped LAM shows a slight shift in UV cutoff wavelength. This was attributed due to incorporation of the dopant and showed broad transmission in UV region. So these materials can be used in the ultraviolet region for the device applications. The dependence of the optical absorption coefficient with the photon energy helps to study the band structure and the type of transition of electrons [18]. The value of band gap energy was estimated from the graph plotted between photon energy h
and ( h )2 by
extrapolating the linear portion of the curve to zero absorption as shown in the Fig. S4 (see supplementary material). Here
is the absorption coefficient and h the photon energy in eV.
The band gap energy calculated was about 3.45 eV for the Cd2+ doped LAM crystal. As a consequence of wide band gap, the crystal under study has relatively larger in the visible region [19]. The internal efficiency of the device also depends upon the absorption coefficient. Hence by tailoring the absorption coefficient and tuning the band gap of the material, one can achieve the suitable material for fabricating various layers of the optoelectronic devices as per the requirements. 3.5. SHG Measurements The second harmonic generation property of the grown crystals was investigated using a Q-switched Nd: YAG laser by Kurtz and perry technique [20]. The sample was ground into very fine powder and tightly packed in a micro capillary tube. Then it was mounted in the path of Nd: YAG laser beam of energy 1.8 mJ/pulse and pulse width about 8 ns with a repetition rate of 10 Hz was allowed to strike the sample cell. The second harmonic signal was about 22.9 mW and 23.1 mW for the undoped and Cd2+ doped LAM crystals respectively which were about 19.4 mW for KDP crystal as a reference material. Hence it was confirmed that the grown
materials have NLO efficiency 1.18 and 1.19 times higher than that of the KDP crystal. The output could be seen as a bright green flash emission from the sample. The green emission confirmed the second harmonic generation in the grown crystals and the metal doping influenced the efficiency of undoped LAM. 3.6. Thermal analysis Simultaneous thermo gravimetric analysis (TG) and differential thermal analysis (DTA) were carried out to study the thermal stability of the grown crystals using NETZSCH-STA409 PL Luxx. Finely powdered crystals were used for the TG/DTA analysis in the temperature range of 30 °C to 1000 °C with a heating rate of 10 C/min in the nitrogen atmosphere. The alumina (Al2O3) was used as a crucible for the sample. The characteristic curve of doped LAM crystal was shown in Fig. S5 (see supplementary material). From the TGA curve, it was observed that the material exhibited number of weight a loss starting at 144 °C and below this temperature, no significant weight loss was observed, which confirmed that the material was very stable with no phase transition up to 144 °C. The nature of weight loss indicated the decomposition point of the material. There were a number of weight losses in the temperature range from 144 °C to 600 °C and it might be due to the liberation of volatile substances in the compounds. In the DTA spectrum an irreversible exothermic peak observed around 152 °C was attributed to the utilization of thermal energy to overcome the bonding between the alaninium cation and the maleate anion during the initial stage of decomposition. But below this temperature no exothermic or endothermic peak was observed. This illustrated the absence of any isomorphic transition. After melting, no characteristic exothermic or endothermic peak was observed which indicated that there was no degradation of the compound above the melting point. Hence, this compound had a good thermal stability up to 152 °C and we can conclude that the Cd2+ doped LAM crystal is suitable for any application up to 152 °C. Also the doped crystal had a good thermal stability, comparable to other organic NLO crystals. The melting point of Cd2+ doped LAM crystal was slightly different compared with the parent material which may be attributed due to the incorporation of impurity (Cd2+) into the lattice of LAM crystal. 3.7. Microhardness Measurements A number of research articles were proved that the material hardness is related to their constituent atoms. So, the microhardness studies were carried out on the grown crystals using Vickers microhardness tester attached with an optical microscope. The Vickers microhardness
number was evaluated from the relation Hv= (1.8544 x p) /d2 kg/mm2 where p was the applied load in kg and d was the diagonal length of the indentation impression in micrometer. The variations of Vickers hardness number with an applied load of undoped and doped LAM crystals were shown in Fig. S6 (see supplementary material). The plot indicated that the hardness of the undoped and doped LAM crystal increased with increasing load was also compared to the parent material and the doped one having a greater hardness value. The higher the hardness values, grater was the stress required to form dislocation and also absence of liquid inclusions. The work hardening coefficient was determined using the relation, p = kdn , k being material’s constant, n the Meyer’s index [21]. The work hardening coefficient ‘n’ is found in undoped and Cd2+ doped LAM crystal by taking the slope of the straight lines of the graph drawn between log p and log d which was shown in Fig. S7 (see supplementary material). According to Onitsch and Hanneman [22,23], n should be between 1 and 1.6 for hard materials and above 1.6 for softer ones. The value of the work hardening coefficient (n) of undoped and Cd2+ doped LAM crystal was found to be more than 1.6. Hence the grown crystals belong to the soft material category and hence fitting as good engineering material for device fabrication. 3.8. Dielectric Measurements The dielectric constant of the undoped and Cd2+ doped LAM crystal were studied at different temperatures using HIOKI 3532 LCR HITESTER in the frequency region 50 Hz to 5 MHz. The samples were pelletized and pellets of uniform dimension were placed between the two copper electrodes. The capacitance and loss were measured in the applied frequency range at different temperatures (40 °C to 80 °C). Figs. 4a and 4b showed the plot of log frequency versus dielectric constant for undoped and doped crystals. The dielectric constant had high values in the lower frequency region and then decreased with the applied frequency in both the crystals. The very high value of dielectric constant at low frequencies might be due to the presence of all the four polarizations namely space charge, orientation, electronic and ionic polarization and its low value at higher frequencies might be due to the loss of significance of these polarizations gradually. From the plot, it was also observed that dielectric constant decreased with increasing frequency and temperature, attributed to space charge polarization mechanism of molecular dipoles near the grain boundary interfaces, which depended on the purity and perfection of the sample [24]. The dielectric loss of undoped and Cd2+ doped LAM showed slight variation in features. Thus, it was concluded that the dielectric nature of the undoped LAM is marginally
altered by the presence of metal dopant. The variation of dielectric loss with log frequency was shown in Figs. 4c and 4d. From the plot it may be concluded that the dielectric loss also decreased with increase in frequency. The low dielectric loss confirmed the purity of the synthesized sample. 4. Conclusions The single crystals of undoped and Cd2+ doped l-alaninium maleate was successfully synthesized by slow evaporation solution growth technique. From the single crystal X-ray diffraction analysis, it was confirmed that the grown single crystals belonged to orthorhombic with the space group P212121 and the lattice parameters were compared. The cell parameter was confirmed and it agreed well with the reported values. EDAX analysis confirmed the presence of impurity in the doped LAM crystal lattice. The functional groups were identified from the FTIR analysis. From UV–Visible spectra, it was observed that there was no absorption in the entire visible region, which is one of the prerequisite optical properties for NLO applications. NLO studies proved that the Cd2+ metal influenced the SHG efficiency of undoped LAM. Thermal analysis revealed that the Cd2+ doping improved the thermal stability of the LAM crystal. Microhardness test of undoped and Cd2+ doped LAM crystals confirmed the good mechanical stability of the materials. The dielectric studies proved that the samples possessed a low dielectric constant and low dielectric loss values at higher frequencies at different temperatures this suggested that the sample possessed an enhanced optical quality with less defects. From these characteristic investigations it can be concluded that the influence of foreign metal ion (Cd2+) improved the growth rate and quality of the LAM crystal and hence the doped crystals can be effectively used as promising NLO material for device fabrication. References [1] M. Kitazawa, R. Higuchi, M. Takahashi, Appl. Phys. Lett. 64 (1994) 2477-2479. [2] W.S. Wang, M.D. Aggarwal, J. Choi, T. Gebre, A.D. Shields, B.G. Penn, D.O. Frazier, J. Cryst. Growth 198 (199) (1999) 578-582. [3] S. Chenthamarai, D. Jayaraman, P.M. Ushasree, K. Meera, C. Subramanian, P. Ramasamy, Mater. Chem. Phys. 64 (2000) 179-183. [4] S. Natarajan, S.A. Martin Britto, E. Ramachandran, Cryst. Growth Des. 6 (2006) 137-140.
[5] R. Sankar, R. Muralidharan, C. M. Raghavan, R.Jayavel, Mater. Chem. Phys.107 (2008) 51-56. [6] C. K. Lashmana Perumal, A. ArulChakkaravarthi, N. BalaMurugan, P. Santhana Raghavan, P. Ramasamy, J. Cryst. Growth 265, (2004) 260. [7] S.R. Marder, J.W.Perry, C.P.Yakymyshyn, Chem. Mater. 6, (1994) 1137-1147. [8] M. Alagar, R.V. Krishnakumar, M. Subha Nandhini S. Natarajan, Acta Cryst. E57 (2001) 855-857. [9] N. Vijayan, G. Bhagavannarayana, R. Ramesh Babu, R. Gopalakrishnan, K.K. Maurya P. Ramasamy, Cryst. Growth Des. 6 (2006) 1542-1546. [10] K. Vasantha, S. Dhanuskodi, J. Cryst. Growth, 263 (2004) 466-472. [11] D. Jaikumar, S. Kalainathan, G. Bhagavanarayana, J. Cryst. Growth 312 (2009) 120-124. [12] S.A. Martin Britto Dhas, G. Bhagavannarayana, S. Natarajan, Open Cryst. J, 1 (2008) 42-45. [13] D. Balasubramanian, R. Jayavel, P. Murugakoothan, Natural Sci. 1(2009) 216-221. [14] M. Victor Antony Raj, J. Madhavan, M. Gulam Mohamed, J. Comput. Method. Mol. Design, 1 (2011) 57-64. [15] M. Vimalan, A. Cyrac Peter, T. Rajesh Kumar, C. Jayasekaran, J. Packiam Julius, P. Sagayaraj, Arch. Phys. Res. 1 (2010) 94-102. [16] M. Victor Antony Raj, J. Madhavan, Arch. Phys. Res. 2 (2011) 160-168. [17] U. Karunanithi, S. Arulmozhi, J. Madhavan, IOSR J. Appl.Phys.1 (2012) 19-23. [18] N. Tigau, V. Ciupinaa, G. Prodana, G.I. Rusub, C. Gheorghies, E. Vasilec, J. Optoelect. Adv. Mater. 6 (2004) 211-217. [19] D.D.O. Eya, A.J. Ekpunobi, C.E. Okeke, Academic Open Internet Journal 17 (2006) 1311-4360. [20] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [21] M.A. Meyers, some aspects of the hardness of metals, Ph.D., Thesis, Dreft, (1951). [22] E.M. Onitsch, Microscopia 2 (1947) 131-134. [23] M. Hanneman, Metall. Manchu. 23 (1941) 135. [24] Christo Balarew, Rumen Duhlev, J. Solid State Chem. 55 (1984) 1-6.
Table captions Table. 1. Single Crystal data of undoped and Cd2+ doped LAM crystals Table. 2. FTIR data comparison of undoped and Cd2+ doped LAM crystals Figure captions Fig.1. FTIR Spectrum of LAM Single Crystal Fig.2. FTIR Spectrum of Cd2+ doped LAM crystals Fig.3. UV-Vis. Spectrum of undoped and Cd2+ doped LAM crystals Fig.4a. Dielectric constant of LAM crystal Fig.4b. Dielectric constant of Cd2+ doped LAM crystals Fig.4c. Dielectric loss of LAM crystal Fig.4d. Dielectric loss of Cd2+ doped LAM crystals Table. 1. Single Crystal data of undoped and Cd2+ doped LAM crystals LAM [8-10]
Cd2+ doped LAM crystals
a = 5.59 Å
a = 5.59 Å
b = 7.38 Å
b = 7.36 Å
c = 23.73 Å
c = 23.66 Å
V= 980 Å3
V= 973 Å3
Orthorhombic
Orthorhombic
P212121
P212121
Table. 2. FTIR data comparison of undoped and Cd2+ doped LAM crystals LAM
Cd2+ doped LAM crystals
ASSIGNMENT
3209
3206
NH3 asymmetric stretching
2937
2934
C-H stretching
1723
1723
COO- asymmetric stretching
1377
1376
COO- symmetric stretching
1260
1260
C-H deformation (overtone)
1104
1106
C - O Stretching
864
863
C - C Stretching
759
759
CH2 rocking
656
654
COO- plane deformation
583
582
COO- wagging
Fig.1. FTIR Spectrum of LAM Single Crystal
Fig.2. FTIR Spectrum of Cd2+ doped LAM crystals
Fig.3. UV-Vis. Spectrum of undoped and Cd2+ doped LAM crystals
Highlights The incorporation Cd2+ is confirmed by single crystal XRD, EDS analysis. Dopant is quite useful since SHG efficiency is enhanced. Optical band gap and transparency of doped crystal is increased in UV analysis. Band gap energies have been estimated. Cd2+ doping enhances the thermal stability of the grown crystal. Mechanical stability is better than pure LAM due to the doping. Cd2+ doping enhances the optical applications of LAM crystal.
S1386-1425(13)00714-2 http://dx.doi.org/10.1016/j.saa.2013.06.100 SAA 10721
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐ lecular Spectroscopy
Received Date: Revised Date: Accepted Date:
20 March 2013 24 June 2013 27 June 2013
Please cite this article as: L. Ruby Nirmala, J. Thomas Joseph Prakash, Investigation on the influence of foreign metal ions in crystal growth and characterization of L -Alaninium Maleate (LAM) single crystals, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.06.100
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on the influence of foreign metal ions in crystal growth and characterization of L -Alaninium Maleate (LAM) single crystals L.Ruby Nirmala, J.Thomas Joseph Prakash* PG and Research Dept. Of Physics, Holy Cross College (Autonomous), (Affiliated to Bharathidasan University) Trichy 620 002, Tamil Nadu, India *PG and Research Dept. of Physics, H.H. The Rajah’s College, (Affiliated to Bharathidasan University) Pudukkottai-1. Tamilnadu, India Abstract A Nonlinear Optical, good quality, single crystals of doped and undoped L-Alaninium Maleate (LAM) were grown by slow evaporation solution growth technique at room temperature. The lattice parameters were analyzed by single crystal X-ray diffraction technique. The identification of Cadmium ion in the doped crystals was done using the EDAX spectrum. The presence of functional group of the dopant with LAM molecule was studied using FTIR spectra. The results of UV-Vis study is used to compare the transparencies of the doped and undoped LAM crystals. The optical band gap energy of the grown crystal was also calculated. The relative second harmonic generation (SHG) efficiency measurement with KDP reference is used to find the incorporation of metal to L-Alaninium Maleate crystals and the parent material. Also the thermal stability of the grown crystals was studied by TGA/DTA spectrum. The mechanical stability of the grown crystals was confirmed through Vickers micro hardness study. By parallel plate capacitor technique, the dielectric response was studied over a wide range of frequencies at different temperatures. The various studies showed the incorporation of the impurity Cd2+ into LAM crystals and the investigations indicated that the impurity played an important role in the changes of the spectral and structural properties of LAM crystals. Key words: NLO material; Growth from solution; Second-harmonic Generation; metal doped LAM. * Corresponding author E-mail: [email protected] Tel: +91 09842470521
1. Introduction NLO materials find
extensive opto electronic applications such as
optical
communication, signal processing, frequency conversion, optical data storage, optical switches etc. [1-3]. These applications depend on the properties such as transparency, refractive index, dielectric constant, thermal, mechanical, photochemical and chemical stability [4]. Among NLO materials, organic NLO materials are generally believed to be more versatile than their inorganic counterparts due to their more favorable nonlinear response [5]. Inorganic materials are also used in these applications due to their high melting point, high mechanical strength and high degree of chemical inertness but their optical nonlinearity is poor. The organic compounds with electron rich (donor) and deficient (acceptor) substituent provide the asymmetric charge distribution in the
– electron system and show large nonlinear optical responses. The family of materials of
an amino acid is extensively researched for NLO activity due to the fact that all the amino acids except glycine contain chiral carbon atom and crystallize in noncentro symmetric space groups which is an essential character of NLO activity [6, 7]. The L-Alanine molecule exists in the cationic form (C3H8NO2+) with a positively charged amino group and is an efficient organic NLO material. This was combined with the maleic acid molecule which is in the mono-ionized state (C4H3NO4-) in order to produce the material L-Alaninium Maleate to challenge the NLO materials already existing. The structure of L-alaninium maleate crystal was already solved by Alagar et.al. [8]. The growth and characterization of L-alaninium maleate crystals were reported by many authors earlier [9 - 15]. The Cu2+, Mg2+ metal ions doped on LAM crystal [16] and the rare earth elements like La3+, Nd3+ doped on LAM crystal [17] were also been reported that the presence of dopants improved the nonlinear optical (NLO) properties of the parent material. Motivated by these considerations an attempt is made to grow another optically transparent material to increase in SHG efficiency, such as a transition metal ion (Cd2+) doped LAM single crystals by the slow evaporation method. The characteristic modification by doping foreign metal ion (Cd2+) on the LAM crystal was studied by different characteristic techniques and the report of these investigations was discussed in this paper. 2. Experimental procedure Material synthesis and crystal growth LAM was synthesized using high purity L-alanine and AR grade Maleic acid from Merck in the stoichiometric ratio1:1with double distilled water. The reactants were thoroughly dissolved
in water and stirred well for about 2 hours to yield a homogeneous solution using a temperature controlled magnetic stirrer. The LAM mixture was synthesized according to the following reactions. C3H7NO2 + C4H4O4
C3H8NO2+ . C4H3O4-
The prepared mixture was dried and a saturated solution was prepared using the synthesized salt. The solution was slightly heated upto an optimum temperature of 55 °C and it was filtered twice to remove the suspended impurities. Then the saturated solution was taken in a crystallizing vessel with a perforated lid in order to control the evaporation rate and kept in undisturbed conditions. After three weeks, transparent single crystals of LAM were harvested from the mother solution. The same procedure was applied to grow metal Cd2+ doped crystals by adding 1mol % of Cadmium Chloride to the saturated LAM solution. The incorporation of dopant into the parent solution has promoted the growth rate and improved the quality of the crystals. Successive recrystallisation was done for purification of the grown crystals and the crystals were found to be transparent and free from defects. The photographs of the ‘as grown’ undoped and Cd2+ doped LAM crystals are shown in Fig. S1(see supplementary material) and Fig. S2 (see supplementary material) respectively. 3. Characterization studies, Results and discussion 3.1. Single crystal X- ray diffraction analysis The undoped and Cd2+ doped LAM crystals were subjected to single crystal X-ray diffraction studies using Enrafnonius CAD4 X-ray diffractometer to determine the unit cell parameters. The obtained lattice parameter values of the undoped and doped crystals were tabulated in Table 1 and the values are well matched with the reported literature [8-10]. It was seen that the grown crystals were crystallized in orthorhombic with the space group P212121, a well known noncentro symmetric space group, thus satisfying the requirements for second order NLO activity.
3.2. EDAX analysis Energy dispersive X-ray analysis is a technique used to identify the elemental composition of a sample. The observed EDS spectrum of Cd2+ doped LAM crystal having the peaks attributed to all the elements at different energies was depicted in Fig. S3 (see supplementary material) which confirmed the incorporation of impurity. All the prominent peaks corresponding to different elements in the sample were seen in the spectrum. 3.3. FTIR spectrum analysis Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify mainly, the functional groups present in organic materials. FTIR analysis provides information about the chemical bonds and molecular structure of a material. In order to qualitatively analyze the presence of functional groups in the grown crystals, the
FT-IR
spectrum were recorded in the range 400 cm-1 to 4000 cm-1 using a KBr pellet on Perkin Elmer RXI FTIR spectrometer and the recorded spectra were shown as Figs. 1 and 2. It was observed that a strong absorption occurred at 3209 and 2934 cm-1 corresponding to the stretching bonds of the NH3+ ion of the amino acid. The strong carbonyl absorption at 1723 cm-1 confirmed the asymmetric stretching of COO- group of the compound.
The absorption bands at about
1376 cm-1 and at 1260 cm-1 may be assigned to the C-H deformation in CH3. A strong bond arising from C–COO- vibration is observed at 1215 cm-1 [10]. The absorption peaks at about 759 and at 582cm-1 may be assigned to the rocking of CH2 group and wagging of COO- group respectively. The characteristic vibrations establishing the identity of all the functional groups present in the compounds were presented in Table 2. The presence of additional peaks in the lower frequency region around 409 cm-1 and 902 cm-1 in the doped spectra may be due to the presence of Cadmium in the coordination sphere. Although the spectrum of Cd2+ doped LAM provides similar features as that of undoped LAM, there is a slight shifting observed for all the peaks suggesting a wide range of interactions for the functional groups. It was also observed that there was broadening or narrowing of some absorption peaks in the FTIR spectrum of doped LAM as that of undoped LAM and this may be due to the incorporation of Cd2+ in the lattice of LAM.
3.4. UV-Visible spectrum analysis The optical transmittance spectrum of undoped and Cd2+ doped LAM samples were recorded using a Perkin-Elmer Lambda 35 spectrophotometer in the range of 200 - 1100 nm. The recorded spectra of the grown crystals were shown as Fig. 3. From the spectrum it was observed that both the crystals showed very high transmission at entire IR and visible regions. This showed the optical transparency of the grown materials. For the undoped LAM the UV cutoff wavelength is found at 340 nm and the doped LAM shows a slight shift in UV cutoff wavelength. This was attributed due to incorporation of the dopant and showed broad transmission in UV region. So these materials can be used in the ultraviolet region for the device applications. The dependence of the optical absorption coefficient with the photon energy helps to study the band structure and the type of transition of electrons [18]. The value of band gap energy was estimated from the graph plotted between photon energy h
and ( h )2 by
extrapolating the linear portion of the curve to zero absorption as shown in the Fig. S4 (see supplementary material). Here
is the absorption coefficient and h the photon energy in eV.
The band gap energy calculated was about 3.45 eV for the Cd2+ doped LAM crystal. As a consequence of wide band gap, the crystal under study has relatively larger in the visible region [19]. The internal efficiency of the device also depends upon the absorption coefficient. Hence by tailoring the absorption coefficient and tuning the band gap of the material, one can achieve the suitable material for fabricating various layers of the optoelectronic devices as per the requirements. 3.5. SHG Measurements The second harmonic generation property of the grown crystals was investigated using a Q-switched Nd: YAG laser by Kurtz and perry technique [20]. The sample was ground into very fine powder and tightly packed in a micro capillary tube. Then it was mounted in the path of Nd: YAG laser beam of energy 1.8 mJ/pulse and pulse width about 8 ns with a repetition rate of 10 Hz was allowed to strike the sample cell. The second harmonic signal was about 22.9 mW and 23.1 mW for the undoped and Cd2+ doped LAM crystals respectively which were about 19.4 mW for KDP crystal as a reference material. Hence it was confirmed that the grown
materials have NLO efficiency 1.18 and 1.19 times higher than that of the KDP crystal. The output could be seen as a bright green flash emission from the sample. The green emission confirmed the second harmonic generation in the grown crystals and the metal doping influenced the efficiency of undoped LAM. 3.6. Thermal analysis Simultaneous thermo gravimetric analysis (TG) and differential thermal analysis (DTA) were carried out to study the thermal stability of the grown crystals using NETZSCH-STA409 PL Luxx. Finely powdered crystals were used for the TG/DTA analysis in the temperature range of 30 °C to 1000 °C with a heating rate of 10 C/min in the nitrogen atmosphere. The alumina (Al2O3) was used as a crucible for the sample. The characteristic curve of doped LAM crystal was shown in Fig. S5 (see supplementary material). From the TGA curve, it was observed that the material exhibited number of weight a loss starting at 144 °C and below this temperature, no significant weight loss was observed, which confirmed that the material was very stable with no phase transition up to 144 °C. The nature of weight loss indicated the decomposition point of the material. There were a number of weight losses in the temperature range from 144 °C to 600 °C and it might be due to the liberation of volatile substances in the compounds. In the DTA spectrum an irreversible exothermic peak observed around 152 °C was attributed to the utilization of thermal energy to overcome the bonding between the alaninium cation and the maleate anion during the initial stage of decomposition. But below this temperature no exothermic or endothermic peak was observed. This illustrated the absence of any isomorphic transition. After melting, no characteristic exothermic or endothermic peak was observed which indicated that there was no degradation of the compound above the melting point. Hence, this compound had a good thermal stability up to 152 °C and we can conclude that the Cd2+ doped LAM crystal is suitable for any application up to 152 °C. Also the doped crystal had a good thermal stability, comparable to other organic NLO crystals. The melting point of Cd2+ doped LAM crystal was slightly different compared with the parent material which may be attributed due to the incorporation of impurity (Cd2+) into the lattice of LAM crystal. 3.7. Microhardness Measurements A number of research articles were proved that the material hardness is related to their constituent atoms. So, the microhardness studies were carried out on the grown crystals using Vickers microhardness tester attached with an optical microscope. The Vickers microhardness
number was evaluated from the relation Hv= (1.8544 x p) /d2 kg/mm2 where p was the applied load in kg and d was the diagonal length of the indentation impression in micrometer. The variations of Vickers hardness number with an applied load of undoped and doped LAM crystals were shown in Fig. S6 (see supplementary material). The plot indicated that the hardness of the undoped and doped LAM crystal increased with increasing load was also compared to the parent material and the doped one having a greater hardness value. The higher the hardness values, grater was the stress required to form dislocation and also absence of liquid inclusions. The work hardening coefficient was determined using the relation, p = kdn , k being material’s constant, n the Meyer’s index [21]. The work hardening coefficient ‘n’ is found in undoped and Cd2+ doped LAM crystal by taking the slope of the straight lines of the graph drawn between log p and log d which was shown in Fig. S7 (see supplementary material). According to Onitsch and Hanneman [22,23], n should be between 1 and 1.6 for hard materials and above 1.6 for softer ones. The value of the work hardening coefficient (n) of undoped and Cd2+ doped LAM crystal was found to be more than 1.6. Hence the grown crystals belong to the soft material category and hence fitting as good engineering material for device fabrication. 3.8. Dielectric Measurements The dielectric constant of the undoped and Cd2+ doped LAM crystal were studied at different temperatures using HIOKI 3532 LCR HITESTER in the frequency region 50 Hz to 5 MHz. The samples were pelletized and pellets of uniform dimension were placed between the two copper electrodes. The capacitance and loss were measured in the applied frequency range at different temperatures (40 °C to 80 °C). Figs. 4a and 4b showed the plot of log frequency versus dielectric constant for undoped and doped crystals. The dielectric constant had high values in the lower frequency region and then decreased with the applied frequency in both the crystals. The very high value of dielectric constant at low frequencies might be due to the presence of all the four polarizations namely space charge, orientation, electronic and ionic polarization and its low value at higher frequencies might be due to the loss of significance of these polarizations gradually. From the plot, it was also observed that dielectric constant decreased with increasing frequency and temperature, attributed to space charge polarization mechanism of molecular dipoles near the grain boundary interfaces, which depended on the purity and perfection of the sample [24]. The dielectric loss of undoped and Cd2+ doped LAM showed slight variation in features. Thus, it was concluded that the dielectric nature of the undoped LAM is marginally
altered by the presence of metal dopant. The variation of dielectric loss with log frequency was shown in Figs. 4c and 4d. From the plot it may be concluded that the dielectric loss also decreased with increase in frequency. The low dielectric loss confirmed the purity of the synthesized sample. 4. Conclusions The single crystals of undoped and Cd2+ doped l-alaninium maleate was successfully synthesized by slow evaporation solution growth technique. From the single crystal X-ray diffraction analysis, it was confirmed that the grown single crystals belonged to orthorhombic with the space group P212121 and the lattice parameters were compared. The cell parameter was confirmed and it agreed well with the reported values. EDAX analysis confirmed the presence of impurity in the doped LAM crystal lattice. The functional groups were identified from the FTIR analysis. From UV–Visible spectra, it was observed that there was no absorption in the entire visible region, which is one of the prerequisite optical properties for NLO applications. NLO studies proved that the Cd2+ metal influenced the SHG efficiency of undoped LAM. Thermal analysis revealed that the Cd2+ doping improved the thermal stability of the LAM crystal. Microhardness test of undoped and Cd2+ doped LAM crystals confirmed the good mechanical stability of the materials. The dielectric studies proved that the samples possessed a low dielectric constant and low dielectric loss values at higher frequencies at different temperatures this suggested that the sample possessed an enhanced optical quality with less defects. From these characteristic investigations it can be concluded that the influence of foreign metal ion (Cd2+) improved the growth rate and quality of the LAM crystal and hence the doped crystals can be effectively used as promising NLO material for device fabrication. References [1] M. Kitazawa, R. Higuchi, M. Takahashi, Appl. Phys. Lett. 64 (1994) 2477-2479. [2] W.S. Wang, M.D. Aggarwal, J. Choi, T. Gebre, A.D. Shields, B.G. Penn, D.O. Frazier, J. Cryst. Growth 198 (199) (1999) 578-582. [3] S. Chenthamarai, D. Jayaraman, P.M. Ushasree, K. Meera, C. Subramanian, P. Ramasamy, Mater. Chem. Phys. 64 (2000) 179-183. [4] S. Natarajan, S.A. Martin Britto, E. Ramachandran, Cryst. Growth Des. 6 (2006) 137-140.
[5] R. Sankar, R. Muralidharan, C. M. Raghavan, R.Jayavel, Mater. Chem. Phys.107 (2008) 51-56. [6] C. K. Lashmana Perumal, A. ArulChakkaravarthi, N. BalaMurugan, P. Santhana Raghavan, P. Ramasamy, J. Cryst. Growth 265, (2004) 260. [7] S.R. Marder, J.W.Perry, C.P.Yakymyshyn, Chem. Mater. 6, (1994) 1137-1147. [8] M. Alagar, R.V. Krishnakumar, M. Subha Nandhini S. Natarajan, Acta Cryst. E57 (2001) 855-857. [9] N. Vijayan, G. Bhagavannarayana, R. Ramesh Babu, R. Gopalakrishnan, K.K. Maurya P. Ramasamy, Cryst. Growth Des. 6 (2006) 1542-1546. [10] K. Vasantha, S. Dhanuskodi, J. Cryst. Growth, 263 (2004) 466-472. [11] D. Jaikumar, S. Kalainathan, G. Bhagavanarayana, J. Cryst. Growth 312 (2009) 120-124. [12] S.A. Martin Britto Dhas, G. Bhagavannarayana, S. Natarajan, Open Cryst. J, 1 (2008) 42-45. [13] D. Balasubramanian, R. Jayavel, P. Murugakoothan, Natural Sci. 1(2009) 216-221. [14] M. Victor Antony Raj, J. Madhavan, M. Gulam Mohamed, J. Comput. Method. Mol. Design, 1 (2011) 57-64. [15] M. Vimalan, A. Cyrac Peter, T. Rajesh Kumar, C. Jayasekaran, J. Packiam Julius, P. Sagayaraj, Arch. Phys. Res. 1 (2010) 94-102. [16] M. Victor Antony Raj, J. Madhavan, Arch. Phys. Res. 2 (2011) 160-168. [17] U. Karunanithi, S. Arulmozhi, J. Madhavan, IOSR J. Appl.Phys.1 (2012) 19-23. [18] N. Tigau, V. Ciupinaa, G. Prodana, G.I. Rusub, C. Gheorghies, E. Vasilec, J. Optoelect. Adv. Mater. 6 (2004) 211-217. [19] D.D.O. Eya, A.J. Ekpunobi, C.E. Okeke, Academic Open Internet Journal 17 (2006) 1311-4360. [20] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798. [21] M.A. Meyers, some aspects of the hardness of metals, Ph.D., Thesis, Dreft, (1951). [22] E.M. Onitsch, Microscopia 2 (1947) 131-134. [23] M. Hanneman, Metall. Manchu. 23 (1941) 135. [24] Christo Balarew, Rumen Duhlev, J. Solid State Chem. 55 (1984) 1-6.
Table captions Table. 1. Single Crystal data of undoped and Cd2+ doped LAM crystals Table. 2. FTIR data comparison of undoped and Cd2+ doped LAM crystals Figure captions Fig.1. FTIR Spectrum of LAM Single Crystal Fig.2. FTIR Spectrum of Cd2+ doped LAM crystals Fig.3. UV-Vis. Spectrum of undoped and Cd2+ doped LAM crystals Fig.4a. Dielectric constant of LAM crystal Fig.4b. Dielectric constant of Cd2+ doped LAM crystals Fig.4c. Dielectric loss of LAM crystal Fig.4d. Dielectric loss of Cd2+ doped LAM crystals Table. 1. Single Crystal data of undoped and Cd2+ doped LAM crystals LAM [8-10]
Cd2+ doped LAM crystals
a = 5.59 Å
a = 5.59 Å
b = 7.38 Å
b = 7.36 Å
c = 23.73 Å
c = 23.66 Å
V= 980 Å3
V= 973 Å3
Orthorhombic
Orthorhombic
P212121
P212121
Table. 2. FTIR data comparison of undoped and Cd2+ doped LAM crystals LAM
Cd2+ doped LAM crystals
ASSIGNMENT
3209
3206
NH3 asymmetric stretching
2937
2934
C-H stretching
1723
1723
COO- asymmetric stretching
1377
1376
COO- symmetric stretching
1260
1260
C-H deformation (overtone)
1104
1106
C - O Stretching
864
863
C - C Stretching
759
759
CH2 rocking
656
654
COO- plane deformation
583
582
COO- wagging
Fig.1. FTIR Spectrum of LAM Single Crystal
Fig.2. FTIR Spectrum of Cd2+ doped LAM crystals
Fig.3. UV-Vis. Spectrum of undoped and Cd2+ doped LAM crystals
Highlights The incorporation Cd2+ is confirmed by single crystal XRD, EDS analysis. Dopant is quite useful since SHG efficiency is enhanced. Optical band gap and transparency of doped crystal is increased in UV analysis. Band gap energies have been estimated. Cd2+ doping enhances the thermal stability of the grown crystal. Mechanical stability is better than pure LAM due to the doping. Cd2+ doping enhances the optical applications of LAM crystal.