Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications

Arabian Journal of Chemistry (2017) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications Sahil Goel a, Nidhi Sinha b, Harsh Yadav a, Abhilash J. Joseph a, Abid Hussain a, Binay Kumar a,* a b

Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India Department of Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India

Received 14 December 2016; accepted 12 March 2017

KEYWORDS Nonlinear optical materials; Dye-doped crystals; Optical properties; Piezoelectricity; Mechanical properties

Abstract Single crystals of pure and xylenol orange (XO; C31H32N2O13S) dye doped (0.01, 0.05 and 0.1 mol%) ammonium dihydrogen phosphate (ADP; NH4H2PO4) were grown by slow evaporation method with the vision to improve the properties of pure ammonium dihydrogen phosphate crystal. The theoretical morphology of the grown crystals was drawn using Bravais–Friedel–Don nay–Harker (BFDH) law. The selective nature of xylenol orange dye to selectively stain the particular growth sectors of ADP crystal was studied. The structural analysis of as grown crystals was carried out using powder XRD study. The identification of the functional groups present in the ADP material was done using Fourier transform infrared (FTIR) spectroscopy. The linear optical study on pure and dye doped crystals was carried out using UV–vis–NIR spectroscopy. The optical band gap, extinction coefficient, refractive index and optical conductivity were calculated using the transmittance spectra for all the samples. In photoluminescence studies, the blue emission intensity got quenched and an orange emission at 597 nm was seen as a result of XO doping. The thermal stability and decomposition temperature of ADP crystal were found to decrease as an effect of dye doping. The piezoelectric charge coefficient, SHG conversion efficiency, mechanical strength and wettability were also enhanced as a result of XO dye doping. Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction * Corresponding author. E-mail address: [email protected] (B. Kumar). Peer review under responsibility of King Saud University.

Production and hosting by Elsevier

In recent years, the need for fast data retrieving, amplifying, modulating, transforming and transmitting a signal by optical techniques has inspired many researchers to grow new nonlinear optical (NLO) materials with exceptionally high optical transparency in the ultraviolet to near IR spectral range (Nie, 1993; Nitti et al., 1993). The NLO active materials extend the applications of the lasers, which include second harmonic generation (SHG), mode-locking and Q-switching (Nie,

http://dx.doi.org/10.1016/j.arabjc.2017.03.003 1878-5352 Ó 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

2 1993; Nitti et al., 1993; Garmire, 2013). The crystals have been observed to exhibit optical nonlinearities at a wavelength from infrared to UV and also used to generate THz radiation (Garmire, 2013). The ammonium dihydrogen phosphate (NH4H2PO4; ADP) and potassium dihydrogen phosphate (KH2PO4; KDP) crystals find their application in optical storage devices, in modern optoelectronics and as monochromators for X-ray fluorescence analysis (Ren et al., 2008; Xu and Xue, 2006). Such excellent applications of ADP and KDP crystals encourage the researchers to grow large plates of these crystals for the fabrication of electro-optic switches and frequency convertors. Many reasonable studies have been reported by researchers on pure KDP and ADP crystals (Xu and Xue, 2006; Yokotani et al., 1986; Takubo and Makita, 1989; Reintjes and Eckardt, 1977). The unremitting interest shown by researchers to ADP and KDP crystals is due to their unique low temperature antiferroelectric, piezoelectric, NLO and electro-optic properties. The orientational dependences of piezoelectric charge coefficients and SHG conversion efficiency depend on the anisotropy of the crystal (Aleshin and Raevski, 2013; Sapaev et al., 2003). The mechanical hardness of ADP crystal is anisotropic in nature (Anbukumar et al., 1987). In order to search new NLO materials, it becomes necessary to tailor the physical properties of the crystals by addition of suitable dopants and by introduction of intrinsic defects to optimize their applicability for the future aspects. In past, many scientists have carried out an extensive research on the growth and characterizations of large size ADP and KDP crystals doped with some inorganic and organic dopants (Dhanaraj et al., 2008; Hasmuddin et al., 2014; Hudson et al., 2014; Rajesh et al., 2010, 2013; DuVarney and Kohin, 1968; Pritula et al., 2001; Shaikh et al., 2015; Meena and Mahadevan, 2008; Bhagavannarayana et al., 2006). Xylenol orange (XO; 3,30 -bis[N,N-di(carboxymethyl)aminome thyl]-o-cresolsulfonephthalein; C31H32N2O13S) is a water-soluble organic dye, which belongs to triphenylmethane (TPM) group. XO dye is widely used in analytical chemistry for the spectrophotometric determination of various metallic ions in aqueous solution, as XO dye forms stable complexes with numerous transition metals (Gupta, 1974; Vandersteen et al., 2004). Dyeing process is a convenient way to introduce the NLO active chromophores in the lattice of single crystal, which promotes the solid state photonic device applications (Pan et al., 1996). As a result of intrinsic polarization, high thermal conductivity, anisotropy and less scattering centers present in the single crystals, crystal becomes an attractive host matrix for dye molecules (Rifani et al., 1995). Dyes lased in liquid phase (dyes in organic solvent) serve as a perfect solution to avoid dye destruction. However, liquid dye lasers suffer from major drawbacks such as inconveniency in disposal of large organic liquid solutions, technical complexities in handling, cleaning and maintaining bulky cells (Costela et al., 2013). These problems can be taken care of by designing compact and easy to handle solid matrices based dye laser devices. Over the past years, great enhancements in the efficiency of SSDL have been achieved as a result of organic dye inclusion in various solid matrixes (Mauri and Moret, 2000; Yariv et al., 2001; Ashwell et al., 1995). Organic dye incorporated into the host matrix of alumina gels, xerogels, silica gels and organic polymers was being used as laser gain media for solid state dye laser (SSDL) devices (Hermes et al., 1993; Amat-Guerri et al., 1993; Reisfeld et al., 1997; Yariv and Reisfeld, 1999). However, the dye inclusion into the polymer host leads to thermal degradation of dye (Costela et al., 2013). Thus, the inorganic crystals doped with organic dye can serve as a promising alternative material for laser working media because of their reduced scattering, intrinsic polarization and high thermal stability (Benedict et al., 2003). Dye molecules in doped crystal provide an alluring option for solid state laser gain media (Kahr and Shtukenberg, 2016). Electrically pumped laser devices based on dye doped organic crystals were found to be more efficient due to tunably high luminescence and high mobility characteristics of dye doped crystals (Kuehne and Gather, 2016). H. Wang and his

S. Goel et al. coworkers have grown large size distyrylbenzene crystals doped with tetracene and pentacene for light emitting diodes, transistors and electrically pumped laser applications (Wang et al., 2009). The electrostatic potential, degree of protonation, crystal surface conditions, and steric exclusion of the dye molecules play an important role for the inclusions of dyes in the crystal planes (Kahr and Shtukenberg, 2016). The striking pattern of the dye molecules on the crystal plane is because of the anisotropic effect of the crystal faces (Lovell et al., 1999). Pritula et al. have observed that the xylenol orange dye inclusion takes place in prismatic {1 0 0} growth sectors of KDP crystals (Pritula et al., 2009). The physical and optical properties of the CV-doped KDP crystals got enhanced along with higher crystalline perfection in comparison with pure KDP (Rajesh et al., 2014). The SHG efficiency of KDP single crystal got enhanced by dye doping such as methyl orange, Rhodamine B, amaranth, and xylenol orange (Pritula et al., 2009; Kumaresan et al., 2008). High transmittance, more efficient luminescence, large dielectric constant and good laser stability were demonstrated for CV dye doped potassium acid phthalate crystal (Rao et al., 2016). In our previous paper (Goel et al., 2016), increase in optical transmittance, mechanical strength and piezoelectric coefficient was reported as a result of crystal violet dye doping in ADP crystal. In the literature, there is no report available on xylenol orange (XO) dye doped ADP single crystals. The organic crystalline materials have higher optical nonlinearity than inorganic materials (Monaco et al., 1987). The inorganic materials possess high mechanical strength, high thermal stability and poor nonlinearity. Keeping this in view, our main aim in the present report was to combine thermal stability and mechanical strength of inorganic ADP matrix with high second order nonlinear optical susceptibility and chemical flexibility of organic (xylenol orange) dye so that it can be successfully employed as a working media for SSDL. In this work, we have reported the effect of XO dye doping on the structural (powder XRD), spectral (FTIR), optical (UV–vis–NIR, PL), thermal (TG/DTA), piezoelectric, mechanical (Vickers microhardness) and nonlinear optical (SHG) properties of the grown ADP crystals. The morphological studies of the pure and XO dye doped ADP crystals were examined using BFDH law. The influence of different dye concentrations in the solution on the color of doped ADP crystals was investigated and the obtained coloring pattern has been discussed in detail.

2. Experimental procedure 2.1. Crystal growth and solubility The commercially available (Merck; AR grade) xylenol orange (XO) dye and ammonium dihydrogen phosphate (ADP) salt were used for crystal growth. A clean beaker was filled with 50 ml of double distilled water (18.2 MX cm resistivity) and measured quantity of the ADP and XO dye (0.0 mol%, 0.01 mol%, 0.05 mol% and 0.1 mol%) was added till supersaturation for different temperatures. The solubility curves of pure and XO dye doped ADP in the temperature range 35– 70 °C in steps of 5 °C are illustrated in Fig. 1. The solubility of XO doped ADP got slightly increased in comparison with pure ADP. Similar results were observed by Rajesh et al. for CV doped KDP (Rajesh et al., 2014). The homogenous solution of pure ADP was prepared by dissolving 25 g of ammonium dihydrogen phosphate in 50 ml of double distilled water at 35 °C. The above prepared solution was stirred continuously for 1 day using a magnetic stirrer to obtain a homogenous and transparent solution. The homogenous solution was filtered using micro pore filter paper and was kept for evaporation in a clean beaker. Now a perforated

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

Xylenol orange doped ADP single crystal

3 et al., 2016a,b). Fig. 2 displays the photograph of as grown pure and XO dye doped ADP crystals. 2.2. Morphological studies and distribution of dye in different crystallographic planes

Fig. 1 Solubility curve of pure ADP, 0.01 mol% XO dye doped ADP (XADP-1), 0.05 mol% XO dye doped ADP (XADP-2) and 0.1 mol% XO dye doped ADP (XADP-3).

aluminum foil paper was used to cover the beaker and the covered beaker was kept in a constant temperature oil bath for controlled evaporation at 35 °C (±0.1 °C). Transparent colorless pure ADP crystals were collected after a time span of 15 days. For the growth of XO dye doped single crystals, 0.01, 0.05 and 0.1 mol% of XO dye were added as dopant to the solution containing 25 g of ADP salt in 50 ml of double distilled water. The same growth parameters as described above were followed to obtain 0.01 mol% (XADP-1), 0.05 mol% (XADP-2) and 0.1 mol% (XADP-3) XO doped ADP crystals. Large size XO doped ADP crystals with good transparency were obtained from the aqueous solution in a lesser time of about 12 days. A similar growth enhancing effect was also identified due to dye doping in several other single crystals (Goel et al., 2016; Bhandari et al., 2014; Yadav

Fig. 2

The theoretical modeling of the crystal morphology is important for efficient device applications. In the case of pharmaceutical products, powder density, optimum packing, etc., the advanced prediction of the crystal morphology is needed. The accurate prediction of the crystal morphology saves time and helps to avoid technical troubleshooting of the crystallization systems (Yadav et al., 2016a). Due to the anisotropic response of the crystal planes, the various physical and chemical properties of the crystal vary along different directions (Liu et al., 1995; Rohl, 2003; Mou et al., 2012). As a result of different growth rates associated with different crystal planes, the crystals are found to be developed with a particular morphology. On the basis of geometrical interpretation, Bra vais–Friedel–Donnay–Harker (BFDH) law is used to predict the crystal morphology. BFDH law states that the growth rate of a particular (h k l) plane is inversely proportional to the interplanar spacing (dhkl) (Docherty et al., 1991). The growth rate (Rhkl) of the (h k l) plane is defined as the growth distance in a unit time interval in a direction normal to the plane. Fig. 3 displays the simulated indexed morphology of as grown ADP crystal using WinXMorph program (Kaminsky, 2005). Table 1 summarizes the growth rate and morphological importance of different planes of pure ADP crystal. The value of the morphological importance (MI) of {0 1 0} plane is larger than that of {1 0 1} plane, which is in proximity with the experimentally observed morphology. The morphology of pure and XO dye doped ADP crystals showed no change except the inclusion of dye in various growth sectors in the XO-doped ADP crystal. Due to the anisotropy of ADP crystal, the crystal faces of ADP that are not related to one another by symmetry must show different affinities to the XO dye molecules in the solution (Kahr and

Photograph of as grown pure and XO dye doped ADP single crystals by slow evaporation technique.

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

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S. Goel et al. dyeing pattern was observed for CV-doped ADP crystal (Goel et al., 2016). 2.3. Characterization techniques

Fig. 3

Indexed morphology of as grown ADP crystal.

Table 1 Morphological importance of the crystal faces of ADP crystal from BFDH law. Faces (hkl)

(1 0 0), (0 1 0), (1 0 1), (0 1 1),

ð 1 0 0Þ ð0  1 0Þ ð1 0  1Þ, ð 10 1Þ, ð1 0 1Þ   ð0 1 1Þ, ð0 1 1Þ, ð0 1 1Þ

dhkl (A˚)

Calculated relative growth rates from BFDH law

Morphological importance (M.I.) by BFDH law

7.484 7.484 5.311 5.311

1.000 1.000 1.409 1.409

1.000 1.000 0.709 0.709

Powder X-ray diffraction (XRD) analysis of the pure and XOdoped ADP crystals was characterized using Bruker D8 Advanced X-ray diffractometer (Cu Ka X-ray source of wavelength 1.5408 A˚). The powdered specimen of pure and XO dye doped ADP samples was examined for FTIR analysis using Perkin Elmer FTIR RXI spectrophotometer in the wavelength range 400–4000 cm1. The optical transmittance spectra of the pure and dye doped specimens were recorded in the UV–vis– NIR range of 200–1100 nm using Perkin-Elmer Lambda-35 UV–vis spectrometer. The PL emission spectra of pure and XO dye doped ADP crystals were characterized using a Varian Cary Eclipse fluorescence spectrophotometer. Thermo gravimetric analysis was performed using Perkin–Elmer diamond TG/DTA analyzer in the nitrogen atmosphere at a heating rate of 1 °C/min in the temperature range from 30 °C to 330 °C. The RT longitudinal piezoelectric charge coefficient (d33) of the grown crystals was measured along [0 1 0] using PM-300 Piezometer system by applying a force of 0.25 N with a tapping frequency of 110 Hz. SHG conversion efficiency of the crystal was measured using Kurtz powder technique employing a Nd:YAG laser (k = 1054 nm). The Vickers microhardness MVH-I tester was used to measure the variation of hardness with respect to the applied indenter load for pure and XO dye doped ADP crystals. 3. Result and discussion 3.1. Powder XRD analysis

Gurney, 2001). Fig. 4(a) depicts the photograph of as grown XO dye doped ADP crystal. From Fig. 4(a) it is clear that the XO dye selectively stains the different growth sectors of ADP single crystal. The incorporation of XO dye into various crystallographic planes was examined by drawing the morphology of XO-doped ADP crystal using the WinXMorph program. The morphology and distribution of dye in ADP crystal are shown in Fig. 4(b). It was found that only the {1 0 0} growth sectors of ADP crystal were selectively stained by the XO dye molecules. Fig. 4(c) shows the fluorescence from the XO-doped crystal, which confirms the inclusion of XO dye in the {1 0 0} growth sectors of ADP crystal. Similar

Grown crystals of pure and doped ADP were crushed and subjected to powder X-ray diffraction (XRD) analysis in the 2h range of 20–80° (at R.T.) with Cu Ka radiation of wavelength 1.5405 A˚. Fig. 5 depicts the XRD patterns of the pure and dye doped samples with reflection peaks being well indexed. The sharp peaks observed in the XRD data confirm that the grown crystals are of good crystallinity. The least-squares refinement of lattice parameters of pure and XO dye doped ADP samples to indexed reflections and to determine the best possible space group was performed using checkcell software (Katrusiak and McMillan, 2004). The unit cell parameters of ADP crystal match well with standard JCPDS card No. 01-078-2414.

Fig. 4 (a) Photograph of xylenol orange in {1 0 0} growth sectors of ADP. (b) Morphological representation of the dye distribution into {1 0 0} sectors of ADP. (c) Fluorescence from the same XO-doped ADP crystal.

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

Xylenol orange doped ADP single crystal

5 ADP crystals in the mid infrared range (400–4000 cm1) are illustrated in Fig. 6. The observed FTIR spectrum matches well with the earlier works reported in the literature (Dhanaraj et al., 2008; Shaikh et al., 2015), which confirms the presence of different functional groups in ADP sample. The FTIR vibrational band assignments of pure ADP and XO doped ADP crystals are represented in Table 3. As such, no noticeable differences in the FTIR spectra of pure and XO doped samples were observed, which can be attributed to the small concentration of XO dopant in ADP. In addition, only minor changes in the peak positions were observed in the spectra of XO doped samples, but no secondary vibrational bands were observed suggesting the presence of XO additive in ADP lattice. 3.3. UV–Vis–NIR spectral analysis

Fig. 5 X-ray powder diffraction patterns of as grown pure and XO doped ADP single crystals.

Table 2 shows the values of refined lattice parameters of the pure and XO dye doped crystals. Doping with XO dye results in the variation of peak intensity but no shift is observed in the position of XRD peaks. The incorporation of heavy XO dye molecules induces strain in the ADP lattice, which results in a slight change in the cell parameter values (Table 2). The pure and XO dye doped ADP crystals belong to the tetragonal crystal system with space group I-42d with a small variation in cell parameter values. Thus, XRD analysis confirms the inclusion of dye molecules in ADP crystal lattice.

The key optical properties of any sample are determined by its extinction coefficient (k), refractive index (n) and absorption coefficient (a). These optical constants play an important role in analysis of laser frequency conversion, designing of optoelectronic devices and new NLO media. For desired optical

3.2. Functional group analysis using FTIR spectroscopy In the process of FTIR spectroscopy, an infrared light signal is passed through the material and the resultant infrared spectrum for the material is recorded. Only those frequencies from the complete incident spectrum (400–4000 cm1) are absorbed, which are equal to characteristic vibrational frequency of the bond. FTIR spectra reveal details about the normal mode of motion, bond strength, bond type and molecular structure of the material. Also the study of intensity, position and shape of the absorption bands in FTIR spectra provides information about several functional groups present in the sample. Using KBr pellet technique, four different pellets of pure and XO dye doped ADP (0.01, 0.05 and 0.1 mol%) mixed with KBr were prepared. The FTIR spectra for pure and XO dye doped

Table 2

Fig. 6 FTIR Spectra of as grown pure and XO dye doped ADP indicating the formation of most of the expected functional groups in crystals; a slight change in the intensity and position of absorption bands confirms the incorporation of XO dye in ADP crystals.

Refined cell parameters of pure and XO dye doped ADP crystals.

Parameters

Pure ADP

XADP-1

XADP-2

XADP-3

Crystal system Space group Cell length, a (A˚) Cell length, b (A˚) Cell length, c (A˚) Cell angle, a (°) Cell angle, b (°) Cell angle, c (°) Volume, (A˚3)

Tetragonal I-42d 7.4844 7.4844 7.5386 90 90 90 422.29

Tetragonal I-42d 7.4871 7.4871 7.5442 90 90 90 422.90

Tetragonal I-42d 7.4996 7.4996 7.5449 90 90 90 424.36

Tetragonal I-42d 7.5008 7.5008 7.5515 90 90 90 424.86

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

6

S. Goel et al. Table 3 Assignment of various functional groups to the characteristic absorption frequencies observed in the FTIR spectra of Pure and XO dye doped ADP crystals. Frequency in wave number (cm1)

Assignments of Functional groups

Pure ADP

XADP-1

XADP-2

XADP-3

3245 3127 2877 2371 1707 1447 1404 1296 1105 916 545 460

3233 3126 2875 2362 1720 1446 1403 1292 1100 917 548 466

3240 3125 2873 2361 1727 1447 1403 1291 1097 917 549 459

3233 3124 2878 2361 1670 1456 1402 1295 1093 922 543 451

Asymmetric stretching mode of NH3+, OAH stretching of hydrogen bonded carboxyl group PAOAH Stretching, NAH Vibration of Ammonium, OAH Stretching (O‚)POAH stretching PAH stretching P‚O stretching Bending vibration of Ammonium (NH4) Bending vibration of Ammonium (NH4) Combination of Asymmetric bending vibration of PO4 with lattice P‚O bending vibration PAOAH bending vibration PO4 bending vibration PO4 bending vibration

applications in industry, the crystal needs to possess high transmittance and lower cutoff wavelength in the range 200– 300 nm (Levanyuk et al., 1979). The transparency cutoff wavelength and optical transmittance window provides information about the various types of optical transitions in any material. The electrons in the r and p orbital are promoted from ground state to the excited states due to absorption of UV and visible light, which provides structural information of the crystal. Fig. 7(a) displays the UV–vis–NIR transmittance spectra of pure and XO dye doped ADP sample in the wavelength range 200–1100 nm at room temperature. The cutoff wavelength for pure ADP is found to be 202 nm. With dye inclusion, the cutoff wavelength increased to 209 nm, 215 nm and 217 nm for XADP-1, XADP-2 and XADP-3, respectively. The optical transparency decreased from 88% (pure) to 85% (XO doped) due to dye doping. The UV–vis–NIR transmittance spectra of XO doped ADP crystals depict three characteristic absorption peaks observed at 270, 440 and 590 nm. These three absorption peaks confirm the presence of XO dye in doped ADP crystals (Pritula et al., 2009). The intensity of the three absorption peaks is the weakest for XADP-1, which increased for XADP2 and is largest for XADP-3. This pattern of the intensity of absorption peaks for dye doped crystals agrees well with the concentration of XO dye doping in ADP lattice. More absorption on the XADP-3 crystal is due to the high incorporation of XO dye in the ADP crystal lattice. Fig. 7(b) depicts the UV–vis–NIR transmittance spectra of XO dye in neutral water solution. In the case of XO dye in aqueous solution, four characteristic absorption peaks were observed at 280 nm, 372 nm, 453 nm and 565 nm. The small absorption peak in the case of XO doped ADP was observed at 590 nm and is red shifted in comparison with that of XO dye in aqueous solution at 565 nm. The two absorption bands of XO doped ADP at 270 nm and 440 nm are blue shifted relative to the absorption peak observed in the case of XO dye in neutral water solution at 280 nm and 453 nm. These red and blue shifts in the absorption bands can be attributed to the dye incorporation into the growth sectors of the doped crystal. The absorption spectrum of the isolated molecules differs from the spectrum of the aggregates. According to the molecular exciton coupling theory, XO dye inclusion into the host ADP crystal can produce splitting of the excited states of the aggregate (Subramony et al., 1997). The changes observed in

Fig. 7 (a) UV–vis–NIR spectrum of pure and XO doped ADP. (b) The transmittance spectrum of XO dye in aqueous solution.

the absorption bands of XO dye in crystal and in neural water solution are mainly due to the change in environmental conditions. The Hamiltonian operator of the isolated chromophore is perturbed by the interactions with the neighboring mole-

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

Xylenol orange doped ADP single crystal

7

Fig. 8 Schematic representation of the inline and parallel transition dipoles depicting the spectral shift (red and blue shifts) based on the molecular exciton coupling theory (Yadav et al., 2016b).

cules. This perturbation leads to exciton splitting of the excited states of the aggregates. The exciton coupling between the neighboring chromophores leads to a shift of the absorption bands either toward lower energies (red shift) or toward higher energies (blue shift), depending on their orientation of molecules next to each other (Marek, 2013). The corresponding aggregates to blue shifts and red shifts are referred to as Haggregates and J-aggregates, respectively (Yadav et al., 2016b). The sign of the difference in dipole moment of dye molecule in the ground and excited state decides the nature of shift. In aggregate molecules, inline transition dipoles refer to the red shift and parallel transition dipoles correspond to the blue shift in the exciton energy level spectrum (Fig. 8). The value of optical band gap (Eg) for pure and XO dye doped ADP single crystal was calculated from the transmittance spectra using the Tauc’s relation: ðahmÞn ¼ Aðhm  Eg Þ, where h is Planck’s constant, A is a constant and m denotes the incident frequency. The value of n is 1/2 for indirectly allowed transition and n is 2 for directly allowed transition (Tauc and Menth, 1972). In Tauc’s relation, a denotes the absorption coefficient (cm1) and is calculated from the forlogð 1 Þ mula a ¼ 2:3026 t T , where t is thickness of the crystal and T is the transmittance of the grown sample. From the Tauc’s relation analysis, it was found that n = 2 was best fitted for the pure and dye doped crystals, which suggests that the grown crystals have direct band gap transitions. The Tauc’s plot was drawn with (ahm)2 on the y-axis and hm along the x-axis for the grown samples (Fig. 9). The linear portion of the absorption edge was extrapolated and the x-intercept of the extrapolated straight line depicted the value of direct band gap (Eg) of the as grown crystals. The value of direct band gap was found to be 6.13 eV, 5.93 eV, 5.77 eV and 5.72 eV for pure ADP, XADP-1, XADP-2 and XADP-3, respectively. Table 4 reports the values of optical transmittance, cutoff wavelengths, optical band gap of pure and XO doped ADP crystals. The transmittance decreases, cutoff wavelength increases and band gap decreases with increase in the amount of XO dye concentration in ADP crystal. The inclusion of XO dye in the ADP crystal causes an increase in the scattering centers and dislocation density, which may be the reason behind the decreased transmittance in dye doped crystals (Chandran et al., 2015).

Fig. 9 Tauc’s plot of (ahm)2 vs. hm and evaluation of direct optical band gap for pure ADP and XO doped ADP crystals; red shift in the cutoff wavelength and decrease in optical band gap due to XO dye doping.

Table 4 The optical transmittance percentages, cutoff wavelengths, direct energy band gap of pure ADP and XO dye doped ADP crystals. Crystals

Optical transmission percentages (%)

Cutoff wavelengths (nm)

Direct energy band gap (eV)

Pure ADP XADP-1 XADP-2 XADP-3

88.0 85.3 85.0 84.2

203 209 215 217

6.13 5.93 5.77 5.72

The reflectance (R) and the absorption coefficient (a) are pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1expðatÞþexpðatÞ related by the following relation: R ¼ 1  1þexpðatÞ

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

8 (Dalal et al., 2014). From the reflectance data, the two optical parameters, refractive indices (n) and extinction coefficients pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3R2 þ10R3 ak (K) were calculated using n ¼ ðRþ1Þ2ðR1Þ and K ¼ 4p , respectively, where k is the wavelength (Dalal et al., 2014). Fig. 10(b) and (c), shows the plot of refractive indices (n) and extinction coefficients (K) against photon energy (hm). The refractive index was found to decrease as a result of XO dye doping in ADP matrix. The optical conductivity (r) for the as grown pure and dye doped crystals was evaluated from refractive index (n) and absorption coefficient (a) as follows: r ¼ acn (Yadav et al., 2016a), where c represents the speed of 4p light. Fig. 10(d) depicts the variation of optical conductivity with respect to hm. From Fig. 10(a), (b) and (d), it can be concluded that the reflectance, extinction coefficient and optical conductivity are enhanced for XO dye doped ADP crystals in the high photon region. 3.4. Photoluminescence properties The photoluminescence (PL) emission from any material is very sensitive to the presence of defects in single crystal and helps to investigate the change in local atomic configuration

S. Goel et al. of crystal structure. The PL emission spectra were recorded on the solid samples of as grown pure and XO dye doped crystals. The reason for choosing solid samples was that the fluorescing dye molecules interact with solvent molecules and hence give low emission in fluid solvents. PL emission spectra for all the samples were recorded at RT in the wavelength range 370–670 nm with an excitation wavelength of 350 nm and are demonstrated in Fig. 11. In the PL analysis of pure ADP, a broad blue emission band consisting of three peaks at 396 nm, 419 nm and 437 nm was observed. For the XO doped ADP samples, the intensity of this broad blue emission band centered at 420–430 nm decreased with increase in dye concentration. In addition to the characteristic blue emission band (420–430 nm) observed for pure ADP, an orange emission band centered at 597 nm (2.08 eV) was also observed for the XO dye doped samples. This orange emission confirms the XO dye inclusion in ADP lattice. Also, the intensity of orange emission increased further with an increase in dye concentration. The orange emission intensity is the largest for XADP-3 crystal, suggesting the largest incorporation of XO dye in the ADP crystal lattice. Quenching of the blue emission intensity can be attributed to the collision between fluorophore (ADP) and quencher (XO dye) molecules (Lakowicz, 1999).

Fig. 10 (a) Variation of reflectance (R) as a function of photon energy (hm). (b) Plot of refractive index (n) with photon energy (hm) on xaxis. (c) Plot of extinction coefficient (K) against photon energy (hm). (d) Variation of optical conductivity (r) as a function of photon energy (hm).

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

Xylenol orange doped ADP single crystal

9 3.5. Thermal analysis

Fig. 11 Photoluminescence spectra of pure ADP and XO dye doped ADP single crystals at room temperature with an excitation wavelength 350 nm; emission of orange fluorescence confirmed xylenol orange dye incorporation into the ADP lattice.

Thermo gravimetric (TG) and differential thermal analysis (DTA) are the useful tools to get information regarding decomposition temperature and phase transition of the crystal system. Fig. 12 shows the TG/DTA spectra of pure and XO dye doped ADP crystals. The curve clearly shows that there is no weight loss up to 168 °C indicating that the pure ADP crystal is thermally stable in the temperature range of RT to 168 °C. Above 168–325 °C, 30% of pure ADP gets decomposed. In case of XO dye doped ADP crystals, no weight loss was observed up to 166.4 °C, 165.7 °C and 163 °C for XAP-1, XADP-2 and XADP-3, respectively. There occurs an endothermic peak (decomposition temperature) at 216.9 °C, 216 °C, 215.5 °C and 213.9 °C for the pure ADP, XADP-1, XADP-2 and XADP-3, respectively, in the DTA curves. When compared to pure ADP, the decomposition temperature is decreased as a result of dye doping. The difference in decomposition temperature observed for the pure and dye doped crystals indicates the incorporation of XO dye molecules into the ADP crystal lattice. The xylenol orange dye molecule has

Fig. 12 TG/DTA curves of (a) pure ADP single crystal, (b) 0.01 mol% XO doped ADP (XADP-1) single crystal, (c) 0.05 mol% XO doped ADP (XADP-2) single crystal and (d) 0.1 mol% XO doped ADP (XADP-3) single crystal.

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

10

S. Goel et al. grown pure and XO dye doped ADP crystals were poled at 1 kV/mm for 2 h. The value of d33 for pure ADP, XADP-1, XADP-2, and XADP-3 was found to be 1.80, 2.54, 2.80 and 2.92 pC/N, respectively, along [0 1 0]. The charge asymmetry developed as a result of dye doping leads to large dipoles on application of mechanical stress, thus explaining the increase in value of d33 for XO doped ADP crystals. 3.7. Nonlinear optical studies

Fig. 13 The variation of Vickers micro-hardness number on the (0 1 0) plane with applied load for pure and XO dye doped ADP crystals; XO dye doping increased the mechanical strength of ADP crystals.

low decomposition temperature (210 °C; Science Material Safety Data Sheet) compared with ADP and hence the decomposition temperature of XO doped ADP single crystals got reduced. The same results have been reported in the past for glycine doped ADP crystal (Rajesh and Ramasamy, 2015) and CV dye doped KAP crystal (Rao et al., 2014). 3.6. Piezoelectric response The lack of center of symmetry is essential for the existence of many useful properties in a single crystal including piezoelectricity (Wojtas´ et al., 2014a). Piezoelectric materials are those which produce electric charge (field) when a mechanical force is applied on them or conversely develop strain under the application of electric field (Wojtas´ et al., 2014b). In either way, there is a linear coupling between stress and the applied field. This makes a piezoelectric crystal useful for various devices such as sensors, actuators and transducers. The as

In NLO active crystals, the induced polarization is nonlinearly related to the applied electric field produced by high power laser system. In particular, doubling of the input frequency is known as SHG. The SHG conversion efficiency of the pure and dye doped crystals was tested using the powder technique developed by Kurtz and Perry (Kurtz, 1968). For SHG testing of the materials, uniform crystalline powder of 63 lm size was packed in a micro-capillary (1.5 mm diameter) tube. A high power Q switched Nd:YAG laser system operated at the repetition rate of 10 Hz with a fundamental beam of wavelength 1054 nm was used for SHG measurement. The input energy of 0.68 J/pulse was allowed to strike normally on the powder sample. The green emission with a wavelength of 532 nm was procured from the samples, which confirmed the SHG. The output energy was measured to be 4.83, 4.89, 5.59 and 5.84 mJ/pulse for pure ADP, XADP-1, XADP-2 and XADP3, respectively. Therefore, the XO doped ADP crystals have shown enhanced SHG efficiency. XO dye molecules exhibit low symmetry representing a polar charge distribution promoting the NLO activity of host (ADP) crystal (Aboulfadl et al., 2012). Moreover, the aligned growth of the XO dye molecules at ADP crystal plane features the nonlinear optical properties (Aboulfadl et al., 2012). Peramaiyan et al. have reported an enhancement in SHG efficiency of L-arginine phosphate crystal by doping with XO dye molecules (Peramaiyan et al., 2013). 3.8. Vickers micro-hardness testing The material hardness is defined as the measure of the resistance offered by the material to local deformation under

Fig. 14 The photograph of honey droplet on (1 0 0) crystal face of pure ADP, XADP-1, XADP-2 and XADP-3 crystals; XO dye doping increased the wettability of ADP crystals.

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

Xylenol orange doped ADP single crystal indentation or scratching (Goel et al., 2016). Vickers microhardness technique was used to test the mechanical strength of as grown pure and dye doped crystals. A diamond indenter with pyramid tip was employed to make indents on the (0 1 0) plane of all the samples. The varying indenter load of values 5 g, 10 g, 20 g, 30 g, 40 g, 50 g, 100 g and 200 g was applied for a constant dwell time of 5 s on (0 1 0) plane of pure and XO doped ADP samples. The value of Vickers microhardness number (Hv; MPa) was evaluated using the formula (Goel et al., 2013): Hv = 1.8544 P/d2, where P represents the applied indenter load in kg and d is the average diagonal length of the indent in mm. Fig. 13 illustrates the plot of harness number (Hv) versus applied load (P) on the (0 1 0) face of pure and doped ADP samples. It can be observed from the figure that the XO doping increases the micro-hardness value of ADP crystal and thus doped ADP crystals show high resistance to motion of dislocation in comparison with pure ADP crystal. The high micro-hardness value for XO dye doped ADP crystals makes them a more suitable candidate than pure ADP from a mechanical point of view for device fabrication. That the micro-hardness value increases with rising applied load up to 20 g load and then decreases with further increase in indenter load up to 40 g is clearly reflected by the Vickers micro-hardness curve depicted in Fig. 13. After 40 g, the harness value shows no remarkable change and attained a constant value. This type of nonlinear variation in hardness with indenter load can be justified on the basis of penetration of the diamond indenter. At lower values of applied indenter load, only the top surface layers of the crystal are penetrated resulting in an increase in hardness with applied loads, which shows the reverse indentation size effect (RISE) (Sahin et al., 2007). However, at higher value of indenter load the indentation depth is large due to which the effect of inner layers is inevitable. The internal stress generated at large load indentation gets released in the form of cracks resulting in a drastic decrease of hardness with applied loads, which shows the normal indentation size effect (ISE) (Sahin et al., 2007). After 40 g, the value of hardness number remains almost constant which is due to the piling-up of the crystal surface. Similar results showing increase in mechanical strength as a result of dye doping had been reported in the literature for various dye doped crystals (Goel et al., 2016; Chandran et al., 2015; Raju et al., 2011). 3.9. Measurement of the contact angle To investigate the effect of XO dye doping on the wettability of the (1 0 0) plane of ADP crystal, we chose honey as a probe liquid. Honey was dropped on the ADP crystal using a micropipet. The honey droplet was pictured using digital camera (Canon SX510HS) featuring a 30 optical zoom. Drop shape analysis for contact angle measurement was done using snake based approach in ImageJ software (Stalder et al., 2006). It was observed that the pure and dye doped ADP crystals exhibited hydrophilic properties in air with honey (Fig. 14). The shape of the droplet we used in this experiment was uninfluenced by gravity. In order to determine more accurate value of contact angle for each crystal, 30 honey droplets were made and the mean value of contact angle was taken. The effect of XO dye doping on contact angle of honey on (1 0 0) crystal plane of ADP is summarized in Table 5. It can be concluded

11 Table 5 Average contact angles of the honey droplets on pure and XO doped ADP crystals. Crystal

Contact angle (o)

Pure ADP XADP-1 XADP-2 XADP-3

81.8 ± 0.9 80.2 ± 0.7 77.4 ± 0.8 61.9 ± 1.7

that XO dye doped ADP crystal wets too much in comparison with pure ADP crystal. The XO dye molecules selectively stain the {1 0 0} growth sectors of ADP crystal, due to which (1 0 0) plane of doped ADP is more dense than that of pure. The high density leads to more attractive force and large surface tension of solids, thus, explaining the decrease in contact angle with increase in dye concentration. 4. Conclusions In summary, pure and XO dye doped ADP single crystals of good transparency were grown by slow evaporation method at RT. BFDH law was successfully employed to understand the crystal morphology of grown crystals. As a result of XO dye doping, the morphology of ADP crystal was not altered and inclusion of XO dye in {1 0 0} growth sectors of ADP crystal was observed. The presence of XO dye in the lattice of ADP was confirmed by the powder XRD, FTIR spectroscopy, UV–vis–NIR and photoluminescence spectral studies. Powder XRD analysis confirmed that the grown pure and dye doped crystals belong to tetragonal structure with non-centrosymmetric space group (I-42d). As a result of low concentration of the dye molecules, no extra absorption bands were observed in the FTIR spectra of dye doped sample. Moreover, only a slight variation in the peak intensity and peak positions in the FTIR spectra was observed as a result of XO dye dopant. From UV–vis–NIR analysis, it can be concluded that transparency decreased (by 3%) and direct energy band gap decreased as a result of dye doping. The value of optical band gap was evaluated to be 6.13, 5.93, 5.77 and 5.72 eV for pure ADP, XADP-1, XADP-2 and XADP-3, respectively. Three absorption peaks were found at 270, 440 and 590 nm in the transmittance spectra of XO dye doped samples, which suggests the presence of XO dye in ADP crystal. In the PL spectra, the blue emission centered at 420–430 nm was quenched and the intensity of orange emission centered at 597 nm was enhanced with an increase in XO dye doping. Increasing XO dye concentration gradually decreased the decomposition temperature of ADP crystal. The piezoelectric charge coefficient value increased with increasing XO dye concentration. The SHG conversion efficiency of the XO doped crystals was larger than that of pure ADP sample and it further increased with an increase in dye concentration. The mechanical hardness of ADP crystal was found to increase as a result of XO dye doping. Increasing XO dye concentration gradually decreased the contact angle of honey with (1 0 0) crystal plane of ADP.

Acknowledgment We are thankful for the financial support received through the DRDO Project (ARMREB/MAA/2015/163) and R & D Grant (Sanction No. RC/2015/9677). Dr. Nidhi Sinha expresses her gratitude to the Principal, SGTB Khalsa College, for encouragement and support for research work. Sahil Goel, Abhilash J. Joseph and Abid Hussain would like to thank CSIR for their Junior Research Fellowship. Harsh Yadav is thankful to UGC for Merit Scholarship.

Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003

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Please cite this article in press as: Goel, S. et al., Optical, piezoelectric and mechanical properties of xylenol orange doped ADP single crystals for NLO applications. Arabian Journal of Chemistry (2017), http://dx.doi.org/10.1016/j.arabjc.2017.03.003