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Studies on the growth, spectral, structural, electrical, optical and mechanical properties of Uronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third-order nonlinear optical applications
Accepted Manuscript Studies on the growth, spectral, structural, electrical, optical and mechanical properties ofUronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third–order nonlinear optical applications A. Silambarasan, M. Krishna Kumar, A. Thirunavukkarasu, I. MD Zahid, R. Mohan Kumar, P.R. Umarani PII: DOI: Reference:
S1386-1425(15)00114-6 http://dx.doi.org/10.1016/j.saa.2015.01.093 SAA 13261
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
11 October 2014 8 January 2015 29 January 2015
Please cite this article as: A. Silambarasan, M. Krishna Kumar, A. Thirunavukkarasu, I. MD Zahid, R. Mohan Kumar, P.R. Umarani, Studies on the growth, spectral, structural, electrical, optical and mechanical properties ofUronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third–order nonlinear optical applications, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.01.093
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Studies on the growth, spectral, structural, electrical, optical and mechanical properties of Uronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third–order nonlinear optical applications A. Silambarasan a, M. Krishna Kumar a, A. Thirunavukkarasu a,b, I. MD Zahida, R. Mohan Kumar a,*, P.R. Umarani c a
Department of Physics, Presidency College, Chennai–600 005, India Department of Physics, S.I.V.E.T College, Chennai–600 073, India c Directorate of Collegiate Education, Govt. of Tamil Nadu, Chennai–600 006, India b
Abstract Organic Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) nonlinear optical single crystal was grown by solution growth technique. The solubility and nucleation studies were performed for UCHBS at different temperatures 30, 35, 40, 45, 50 and 55oC. The crystal structure of UCHBS was elucidated from single crystal X–ray diffraction study. High resolution X–ray diffraction technique was employed to study the perfection and internal defects of UCHBS crystal. Infrared and Raman spectra were recorded to analyze the vibrational behavior of chemical bonds and its functional groups. The physico-chemical changes, stability and decomposition stages of the UCHBS compound were established by TG–DTA studies. The dielectric phenomenon of UCHBS crystal was studied at different temperatures with respect to frequency. Linear optical properties of transmittance, cut-off wavelength, band gap of UCHBS were found from UV-visible spectral studies. Third–order nonlinear optical susceptibility, nonlinear refractive index, nonlinear optical absorption coefficient values were measured by Z–scan technique. The mechanical properties of UCHBS crystal was studied by using Vicker’s microhardness test. The growth features of UCHBS crystal were analyzed from etching studies. Keywords: Crystal growth; Infrared spectroscopy; High-resolution X-ray diffraction; Z-scan technique; 1
*Corresponding author: Dr. R. MOHAN KUMAR Department of Physics Presidency College Chennai-600 005, India Tel: +91-9444600670; Fax: +91-44-28510732 Email: [email protected]
1. Introduction Recently, materials with excellent nonlinear optical (NLO) properties and fast response have attracted much attention for their widespread applications in optical switching, signal processing, optical power limiting and optical communications, optical data storage applications [1–3]. Due to these challenging applications, the growth of optical single crystals is very important for the fabrication of technologically important devices [4–6]. The use of single crystals is clearly noticeable in electronics and optics [7]. The high NLO property is the needed quality for advanced hybrid materials useful in next generation optical technologies. It was found that organic materials show excellent nonlinearity than inorganics. The delocalized π-electron system enhances their asymmetric polarizability and it leads to the microscopic origin of nonlinearity in molecular organic NLO materials [8]. The novel molecular designs with higher third–order nonlinearity and to incorporate them into the devices are the urgent need for optical applications [9–10]. Urea based compounds offered a wide choice of materials and expected to play an important role in future [11]. The acid component 5–Sulfosalicylic acid is an excellent hydrogen donor and hydrogen acceptor and used to design novel crystal with Urea. 5–Sulfosalicylic acid has three potential coordination sites and it can give mono–, di– and tri–anionic ligand species through de-protonation. The presence of large amount of oxygen atoms in the functional groups can be afforded hydrogen bonded associations and used to self-assembly of crystallization [12]. 2
In the present report, Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) crystal was grown from aqueous solution by slow evaporation method. The grown UCHBS crystals were subjected to X-ray diffraction, infrared, Raman, thermal, dielectric, UV-visible spectral studies and mechanical studies. The third order nonlinear optical property of UCHBS crystal studied by employing Z–scan technique and results are presented.
2. Materials and methods The analytical grade starting materials Urea and 5–Sulfosalicylic acid were used for synthesis of UCHBS compound. BRUKER KAPPA APEX II X–ray diffractometer was used to determine the crystal system and lattice parameters of UCHBS crystal at 293 K. High resolution X–ray diffraction curve was recorded by using PANalytical X'PertPRO MRD system with MoKα1 radiation to assess the crystalline perfection. BRUKER IFS FT–IR and Bruker RFS27 spectrophotometers with 100 mW laser excitation were used to record Infrared (IR) and Raman spectrum respectively. TGA–DTA thermogram was traced from RT to 600oC using SDT Q600 V8 instrument. The dielectric measurements were carried out by using HIOKI 3532-50 LCR HITESTER in the frequency range 50 Hz–5 MHz. UV-visible spectrum was recorded for the grown crystal using VARIAN CARRY 5E photospectrometer. Z–scan technique was employed to determine third–order nonlinearity using He–Ne laser source at 632.8 nm. Vicker’s microhardness test was carried out for UCHBS crystal using Shimadzu HMV–2000 Vicker’s pyramidal indenter.
3
3. Experimental 3.1 Synthesis, Solubility, metastable zone width and Crystal Growth Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) compound was synthesized by dissolving equimolar quantity of Urea (6.006 g) and 5-Sulfosalicylic acid (25.422 g) in 100 ml deionized water and stirred. The solution was magnetically stirred about 8 h to achieve homogeneous mixture and synthesized (Fig.1). It was dried at room temperature and synthesized salt was collected. Repeated recrystallization process was adopted for purification of UCHBS salt in aqueous solution and impurities were removed by filtration. The solubility and metastable zone width measurements were carried out in a constant temperature bath with an accuracy ± 0.01ºC. The solubility was determined by gravimetric analysis at different temperatures ranging 30–55ºC. 100 ml of deionized water (solvent) was taken in a flask and UCHBS salt was added upto the saturation point at 30oC. The solution was stirred for about 8 h to achieve homogenization by using an immersible magnetic stirrer and saturated solution was achieved. 10 ml of saturated solution was taken out and poured into the petri dish and dried. Then, the dried UCHBS salt was weighed. The same process was repeated for other temperatures. The solubility of UCHBS is shown in Fig.2. From the plot, it was observed that solubility curve shows a positive gradient which is suitable for slow cooling and evaporation crystal growth methods [13]. The metastable zone width experiment was carried out for UCHBS and metastable zone width was estimated by polythermal method [14]. It is a measure of nucleation temperature at a particular concentration of solute (Fig.2). For finding metastable zone width, newly prepared saturated solution at 30oC was slowly cooled until the first speck appeared and it was instantly recorded as the nucleation temperature. The difference between the saturation and nucleation temperature gives the metastable zone width of the 4
UCHBS in deionized water. The experiment was repeated at different saturation temperatures such as 30, 35, 40, 45, 50 and 55ºC. To grow the good UCHBS crystals, the purified UCHBS salt was dissolved in aqueous solution and saturated at 40oC. The saturated solution was kept at 40oC and maintained the temperature over the growth period. The supersaturated growth solution yielded good quality optical grade UCHBS single crystal in the period of three weeks and collected for characterization. The harvested as-grown UCHBS single crystal with dimension of 16 x 12 x 4 mm3 is shown in Fig.3.
4. Results and discussion 4.1 X–ray diffraction studies The single crystal X–ray diffraction data collection was made for UCHBS crystal at 293 K. It reveals that UCHBS crystal belongs to P1 space group of triclinic crystal system (Table.1). The observed unit cell values are a = 7.219 Å, b = 8.748 Å, c = 10.139 Å, α = 85.504º, β = 71.087º, γ = 68.659º and V = 566.50 Å3 [15]. Figure 4 shows the molecular structure of UCHBS unit molecule. The asymmetric unit of UCHBS consists of one CH5N2O+ cation and one C7H5O6S- anion. The dihedral angle between the benzene ring and the mean plane of the Urea fragment is 76.02 (8)°. In the anion, the carboxyl group makes the dihedral angle of 1.47 (9)° with the benzene ring. The molecular structure confirms that cation and anion are linked by O—H···O and N—H···O hydrogen bonds, and the molecular conformation of the anion is controlled by weak O—H···O hydrogen bond. The study of the defect distribution in the grown crystal is of significant importance because the defects created during growth have severe consequence on the physical properties of the crystal. The degree of crystalline perfection of the 5
UCHBS crystal was characterized by recording high–resolution X–ray diffraction (HRXRD) rocking curve using a multicrystal diffractometer (Fig.5). The absence of additional satellite peaks and the single diffraction curve (DC) peak show that, crystalline perfection of the grown UCHBS crystal is fairly good without having any internal structural grain boundaries. The FWHM (full width at half maximum) of the curve was found to be 116'' arc s which is acceptable for a perfect crystal [16].
4.2 Infrared and Raman spectral analysis The presence of functional groups in the UCHBS compound was confirmed by infrared (IR) and Raman spectral analysis and spectra are shown in Fig.6 and Fig.7 respectively. The assignments of bands observed in the vibration spectra are very useful to understand the chemical structure of UCHBS. The observed IR and Raman bands together with their functional assignments are presented in Table 2. The spectral analysis was performed on the basis of the characteristic vibrations of Urea and 5-sulfosalicylic acid presence in UCHBS compound. C–H Vibrations The C–H stretching vibration of hetero–aromatic structure is usually observed in the region 3200–3000 cm-1. Vibrational peaks at 3136 cm-1 in IR and 3091 cm-1 in Raman spectrum confirmed the presence of aromatic C–H stretching in UCHBS. The substitution sensitive C–H inplane bending vibrations lie in the region 1300–1000 cm-1[17]. The band observed at 1080 cm-1 is assigned to inplane C–H bending in IR and Raman spectra. Bands involving the C–H out of plane vibrations generally occur between 1000–675 cm-1[17] and it was observed at 836 cm-1 in IR spectrum. 6
O–H Vibrations Vibrations due to O–H stretching are sensitive. Hydrogen atom bonded with carboxylic groups display an intermolecular hydrogen bond formation and its vibrational frequency lies in the rage of 3590–2400 cm-1. It was observed at 3406 cm-1 in IR and 3036 cm-1 in Raman spectrum. The FT–IR band identified at 2580 cm-1 is assigned to O–H stretching vibration. The presence of carboxylic and water molecules in UCHBS was confirmed. N–H Vibrations N–H vibrations occur in the region of 3500–3200 cm-1 [18]. The band observed at 3223 cm-1 in IR spectrum is due to N–H stretching vibration. N–H bending vibrations were observed at 1631 cm-1 and 1591 cm-1 in IR and Raman spectra respectively. C–S Vibrations and C–N Vibrations The C–S stretching bands are usually observed in the range of 930–670 cm-1 [19,20] with medium intensity. It was observed at 761 cm-1 in FT-IR and 778 cm-1 in FT-Raman of UCHBS compound. C–N stretching vibrations were observed at 1425 cm-1 in IR spectrum and 1410 cm-1 in Raman spectrum. C=O Vibrations The absorptions are sensitive for both carbon and oxygen atoms of the carbonyl group in the vibrational spectrum. Both have the same amplitudes while it vibrates. Normally carbonyl group vibrations occur in the region 1870–1540 cm-1 [21]. In the present study, the peaks at 1665 cm-1 in IR and 1673 cm-1 in FT-Raman are due to carbonyl C=O stretching vibrations of UCHBS. The C=O bending vibration occurred at 720 cm-1 in the FT-Raman spectrum. 7
C–O Vibrations The C–O stretching vibration usually appears in the region 1300–1200 cm-1 [22]. In the present case, the vibration at 1210 cm-1 in IR and intense bands at 1235, 1207 cm-1 in Raman spectra were observed. C–C Vibrations The vibrational bands observed between 1650 and 1400 cm-1 in benzene derivatives are assigned to C–C ring stretching modes [23]. Accordingly in the present investigation, the carbon–carbon vibrations were observed at 1480 cm-1 in IR and the symmetric C–C stretching was noticed at 1026 cm-1 in FT-Raman spectra. The C–C in- plane vibration was found at 662 cm-1 in IR spectrum. The C–C out of plane and C–C stretching vibrational frequencies were found at 589 and 586 cm-1 in IR and FT-Raman spectra respectively. S=O and S–O Vibrations The sulfonate group vibrations S=O were occurred at 1352, 1157 cm-1 in IR and 1310, 1174 cm-1 in FT-Raman spectra. Also the vibrations found at 934 cm-1 and 883 cm-1 are assigned to S–O vibrations in IR and FT–Raman spectra respectively. 4.3 Thermal analysis Thermogravimetric analysis (TG-DTA) is the measure of the degradation of the compound with an increase of temperature. The experiment was carried out in nitrogen atmosphere with the heating rate of 10 oC/min. The resulting TG-DTA trace of UCHBS is shown in Fig.8. The first decomposition stage of UCHBS was observed between 194.82 oC and 236.49 oC and it was 8
about 6% of mass loss of the compound. Due to the decomposition, NH3 group (6% of mass) expelled from UCHBS. Similarly, a sharp endothermic peak observed at 194.67 oC in DTA curve reveals the melting point of UCHBS. It also indicated the good degree of crystallinity. During the first stage of decomposition, exothermic process was found at 221.82oC in DTA curve. The second decomposition stage was observed between 236.49oC and 302.54oC. It was due to the release of CO2 gas from Urea component of UCHBS and it was about 15 % weight loss from UCHBS. The third weight loss occurred between 302.54 oC and 343.70 oC is due to the release of NO2 gas and it caused about 12% weight loss of UCHBS. From the DTA curve, an exothermic peak observed at 339.76oC is due to the release of NO2 gas. The fourth weight loss occurred in the temperature range between 343.70 oC and 370.43 oC is due to the removal of CO2 gas by the decomposition of sulfosalicylic component and it was about 16% weight loss of UCHBS. The fifth decomposition stage was started at 370.43oC and went upto 461.12oC. It was noted due to the release of SO2 gas from sulfosalicylic component and it was about 23% of weight loss from UCHBS. The residue of UCHBS left at 600ºC was about 29%. From the TG curve, it is concluded that the UCHBS is stable upto 194.8oC. Also, it was observed that, there is no phase transition and weight loss before this temperature. The observed thermal stability of UCHBS is most useful for advanced optical applications. 4.4 Dielectric measurement The dielectric characteristics of the material are important to know the transport phenomena and the lattice dynamics of the crystal. It also gives the information about the nature of atoms, ions, bonding and their polarization mechanism of the material [24]. The crystals with high transparency and large surface defect free were selected and used for dielectric measurements. 9
The selected crystals were polished and silver paste was applied on both sides of the sample for good conduction. The dielectric studies were carried out for UCHBS in the temperature range 313–343 K and the frequency range of 50 Hz–5 MHz. The dielectric constant (εr) was calculated using the relation,
=
(1)
where C is the capacitance, d is the thickness of the crystal, εo is the permittivity of free space, and A is the area of the crystal used. The dielectric loss was calculated using the relation, ε'' = εrD
(2)
where D is the dissipation factor. The response of dielectric constant and dielectric loss as a function of frequency is shown in Fig.9&10. From the plots, it was observed that the dielectric constant and dielectric loss decreases with an increase in frequency and attains saturation at higher frequencies. The dielectric constant and dielectric loss measured at different temperatures almost remains the same at higher frequencies. It indicates that, the temperature variation has no influence on the dielectric property of the material. The low values of dielectric constant and dielectric loss at higher frequencies reveal the quality of grown UCHBS crystal with less defects, which is the desirable property of the materials to be used for various optical and communication applications [25]. Also, the low values of dielectric constant and dielectric loss at higher frequencies are important in the construction of photonic and NLO devices.
4.5 UV–Visible transmission studies Optical devices need crystals with highly transparent over a considerable region of wavelength and defect free [26]. UV–Vis spectrum gives useful information about the structure of the molecules that is the promotion of electrons in σ and π orbits. The thickness of the crystal 10
sample used for this measurement is 1.1 mm. The spectrum was recorded in the range 190-900 nm and shown in Fig.11. It shows good transmission window between 345 to 900 nm, which is most useful for visible region optical applications. The cut–off wavelength of UCHBS crystal was found to be 345 nm. UV-visible study indicates the good optical transmittance in the useful optical region of UCHBS crystal. 4.5.1 Optical band gap and refractive index estimation Optical band gap (Eg) and refractive index (no) of the material are important physical parameters for the optical crystals using in linear and nonlinear optical applications. The band gap energy of the material is very closely related to the material’s atomic and electronic band structures [27]. The energy gap of UCHBS crystal was calculated using the relation, =
λ
(3)
where h is the Planck’s constant, c is the velocity of light and λ is the cut-off wavelength of UCHBS crystal. The optical absorption coefficient (α) was calculated using the relation,
=
. (/)
(4)
where ‘T’ is the transmittance and ‘t’ is the thickness of the crystal. For the direct band gap of material, the absorption coefficient ( ) obeys the relation for high photon energies (ℎυ), ℎ = (ℎ − )/
(5)
where ‘A’ is a constant, ‘Eg’ is the optical band gap, ‘h’ is the Planck’s constant, and ‘υ’ is the frequency of the incident photons [27]. The band gap energy (Eg) was calculated from linear part of the Tauc’s plot drawn between (αhυ)2 and photon energy (hυ) (Fig.12). The band gap energy 11
of UCHBS crystal was found to be 3.46 eV. The refractive index of an optical material can be calculated using transmission spectrum and the reflectance of the crystal may be described in terms of absorption coefficient (α) [28], =1±
"#$%&(#')($%& (') #)*+ (#')
(6)
The refractive index (no) was calculated using the relation, , =
#(-()±√#-/ (-# (-#)
(7)
The calculated refractive index value (no) of UCHBS crystal is 2.63 and it can be used to calculate third-order nonlinear optical susceptibility of the material.
4.6 Z-scan measurement Z-scan technique is an effective and popular technique for characterizing the third-order optical nonlinearity of the materials [29, 30]. Z-scan measurement is a widely used to determine the magnitude and sign of the nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the samples. The measurements were performed in open aperture and closed aperture modes. The measurement in open aperture mode gives the information about the nonlinear absorption coefficient and closed aperture mode helps to calculate the nonlinear refractive index of the material (Fig.13 & 14). He-Ne laser with wavelength of 632.8 nm was used in this experiment. The beam radius of 1 mm was used to scan the sample. Optically polished UCHBS crystal of thickness 1 mm was used in this study. Experiment was performed by placing the sample in the path of the beam at different positions with respect to the focus and the respective light transmission was measured. 12
From the Z-scan data, the difference between the peak and valley transmittances (∆TP-V) can be calculated using the relation [31], ∆12#3 = 0.406(1 − 7).8 |∆φ |
(8)
where ∆φo is the axis phase shift at the focus, S is the linear transmittance aperture and it was calculated using the relation, /
7 = 1 − :;< =− ?/> @ >
(9)
where ra is the radius of aperture and ωa is the beam radius at the aperture. The nonlinear refractive index (n2) was calculated using the relation [32-34], , = BC
∆φA
(10)
A DEFF
where k is the wave number (k=2π/λ), Io is the intensity of the laser beam at the focus (Z=0) and Leff = [1-exp (-αL)]/α is the effective thickness of the sample, α is the linear absorption and L is the thickness of the sample. The nonlinear absorption coefficient (β) can be estimated from the open aperture Z-scan data.
G =
√.∆
(11)
CA DEFF
where, ∆T is peak value at the open aperture Z-scan curve. The value of β is negative for saturable absorption and positive for two photon absorption. The real and imaginary part of the third order nonlinear optical susceptibility (χ(3)) were estimated using [35], : χ() =
HI A J /KA/ K/ L
13
(MN ⁄O)
(12)
QN χ() =
H/A J / KA/λR SL/
(MN ⁄O)
(13)
where, εo is the vacuum permittivity, c is the velocity of light in vacuum. no is the linear refractive index of the sample, and λ is the wavelength of laser beam. The third order nonlinear optical susceptibility was calculated using the relation,
χ() = T( :χ() ) + (QNχ() )
(14)
The calculated third order nonlinear optical susceptibility is given in Table 3. Due to excellent nonlinear response, UCHBS can be a promising material for nonlinear optical device fabrication.
4.7 Microhardness and etching studies Microhardness test is one of the best methods of understanding the mechanical properties of materials such as hardness number, crack length, fracture toughness, brittle index and elastic stiffness constant. Hardness of a material is a measure of resistance its offers to local deformation [36]. In case of UCHBS crystal, loads ranging from 25 to 400 g were used for making indentations, keeping the time of indentation at 10 s for all the cases. The diagonal lengths of the indentation mark and the crack lengths were measured, using a micrometer eyepiece. The Vicker’s hardness value was calculated using, VW =
.X88S 2 Y/
(Z[/NN )
(15)
where, ‘Hv’ is the hardness number in kg/mm2, ‘P’ is the applied load in kg and ‘d’ is the average diagonal length of the indented impressions in ‘mm’. From the Figure 15, it was observed that the hardness value increases with increase in applied load, which is known as reverse indentation size effect (RISE) [37]. It clearly indicates that crystal possesses good mechanical strength. The relation connecting the applied load and diagonal length ‘d’ of the indenter is given by 14
Meyer’s law, P = adn, where ‘n’ is the Meyer’s index which is the measure of ISE and RISE and ‘a’ is the constant. For the normal ISE behavior, the exponent n value is < 2 and also n > 2 for the RISE behavior. When n = 2, the hardness is independent of the applied load, and it is given by Kick’s law. The ‘n’ value of UCHBS crystal was obtained by using linear fit (Fig.16) and it was found to be 0.3173. When the indenter just touches the surface, there is an increase in the hardness value with load, which can be attributed to the work hardening of the surface layers [38-39]. The microhardness value attains nearly a constant due to the rearrangement of dislocations and mutual interaction of dislocations. The result of Vicker’s microhardness test confirms that, UCHBS is classified as soft category material.
The crystallization perfection and growth features can be analyzed from etching studies. The etch patterns on the flat surface of UCHBS crystal were analyzed using water etchant and it was observed by using a high resolution microscope. The rectangular etch pits was observed on the surface of UCHBS crystal as shown in Fig.17. The sharp rectangular etch pits were noticed on the grown crystal for etching time of 10 s and further increase of etching time (15 s), broad rectangular etch pits were observed on the surface of the crystal. For the etching time of >20 s, etch pits were disappeared. This shows the good crystalline nature as well as the regular growth pattern of the UCHBS crystal.
5. Conclusion Optical quality single crystal of UCHBS was grown by slow evaporation method. The solubility and metastable zone width were determined for UCHBS salt in deionized water. The crystal structure and crystalline perfection of UCHBS were analyzed from single crystal and HRXRD studies. The chemical composition of UCHBS was analyzed by FT-IR and 15
FT-Raman spectral studies. Thermal stability of UCHBS compound was estimated using TG-DTA analyses. Low value of dielectric constant and dielectric loss for various temperatures confirmed the quality of the UCHBS crystal. UV–visible studies confirm that grown crystal can be useful for optical applications in the wavelength region 345–900 nm. The optical band gap energy, nonlinear optical refractive index (n2), nonlinear absorption coefficient (β) and thirdorder nonlinear optical susceptibility (χ(3)) were calculated by Z-scan technique. Vicker’s microhardness test reveals that UCHBS belongs to soft category NLO crystal. The etching study revealed the growth mechanism of UCHBS crystal, which found to have layered growth with rectangular etch pits.
16
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[29] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, Opt. Lett. 14 (1989) 955–957. [30] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760–769. [31] T. Kanagasekaran, P. Mythili, P. Srinivasan, A.Y. Nooraldeen, P.K. Palanisamy, R. Gopalakrishanan, Cryst. Growth Des. 8 (2008) 2335–2339. [32] G. AnandhaBabu, P. Ramasamy, Spectrochim. Acta Part A 82 (2011)521–526. [33] A. Subashini, R. Kumaravel, S. Leela, H.S. Evans, D.Sastikumar, K. Ramamurthi, Spectrochim. Acta Part A 78 (2011) 935–941. [34] R. Ashok Kumar, R. Ezhilvizhi, N. Vijayan, G. Bhagavannarayana, D. Rajanbabu, J. Pure Appl. and Indus. Phys. 1 (2010) 61–67. [35] D. Bharath, S. Kalainathan, Spectrochim. Acta Part A 118 (2014) 1098–1105. [36] W. Mott, Micro Indentation Hardness Testing, Butterworths, London, 1956. [37] K. Sangwal, B. Surowska, P. Błaziak, Mater. Chem. Phys. 77 (2002) 511–520. [38] E. M. Onitsch, Mikroskopie 2 (1947) 131–151. [39] P. Vivek, P. Murugakoothan, Optic. Laser Tech. 49 (2013) 288–295.
19
Fig.1 Material synthesis scheme for UCHBS
20
55
Solubility curve Nucleation curve
45
o
Temperature ( C)
50
40 35
M
n zo e l tab s a et
id ew
th
30 25 20 70
80
90
100
110
120
130
140
Concentration (g/100 ml) Fig.2 Solubility and nucleation curves of UCHBS in water solvent
21
Fig.3 Photograph of UCHBS single crystal grown by slow evaporation method
22
Fig.4 The molecular structure of UCHBS unit molecule
23
140000
ΜοΚα1
-1 Diffracted X-ray Intensity (cs )
36" 120000
(110) plane (+,-,-,+)
100000 80000
116"
60000 40000 20000 0 -400
-200
0
200
400
Glancing angle (arcsec) Fig.5 High-resolution X-ray diffraction curve recorded for a typical UCHBS single crystal specimen using (110) diffracting plane
24
1.2
1942
0.8
4000
3500
3000
2500
2000
1500 -1
Wavenumber (cm )
Fig.6 FT-IR spectrum of UCHBS crystal
25
532
662
1000
589
3223 3136
++
1210 1157 1080
1665 1631
2580
0.2
0.0
1480 1425 1352
0.4
934 836 761
0.6
3406
Transmittance (%)
1.0
500
73
0.8
1207 1174
0.7
1235
0.4
1673 1591 1410 1310
0.3 0.2 0.1
1026 883 778 720 586 527 444 251 170
1080
0.5
3091 3036
Raman Intensity
0.6
0.0 4000
3500
3000
2500
2000
1500
1000
Wavelength (nm)
Fig.7 FT-Raman spectrum of UCHBS crystal
26
500
0
o
+ +
80
+
Weight %
Exo
o
+ 302.54
10
TGA DTA
+
221.82 C
70
+
o
C
5
343.70 oC
60
+
0 50
+ 370.43
o
C
40
-5
o
461.12 C
+
30
-10
20 100
200
300 400 o Temperature C
500
Fig.8 TG-DTA curve of UCHBS crystal
27
600
Temperature difference oC/mg
o C o C 4 9 5 4 1. 6. 29 23
o
194.67 C
90
C C
+194.82
33 9. 76
100
15
Endo
o
300
313 K 323 K 333 K 343 K
280 260
Dielectric constant
240 220 200 180 160 140 120 100 80 60 1
2
3
4
5
6
Log f Fig.9 Plot of Dielectric constant vs. Log f of UCHBS crystal
28
7
8
313 K 323 K 333 K 343 K
7
Dielectric Loss
6 5 4 3 2 1 0 1
2
3
4
5
6
Log f Fig.10 Plot of Dielectric loss vs. Log f of UCHBS crystal
29
7
100
Transmittance (%)
80
60
40
20
0
100
200
300
400
500
600
700
800
900
Wavelength (nm)
Fig.11 UV-Visible transmission spectrum of UCHBS crystal
30
0.06
(αhν)
2
(eV/m)
2
0.05 0.04 0.03 0.02 0.01 0.00 Eg= 3.46 eV
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy (eV) Fig.12 Tauc’s plot drawn between (αhυ)2 and photon energy (hυ) for UCHBS crystal 31
0.80
Normalized Transmittance
0.75 0.70 0.65 0.60 0.55 0.50 0.45 -8
-6
-4
-2
0
Z (mm)
32
2
4
6
8
Fig.13 Z-scan plot of UCHBS crystal in open aperture
Normalized Transmittance
0.81
0.80
0.79
0.78
-8
-6
-4
-2
0
Z (mm) 33
2
4
6
8
Fig.14 Z-scan plot of UCHBS crystal in closed aperture
50
2
Hardness number H V (kg/mm )
45 40 35 30 25 20 15 0
50
100
150
200
250
Load P (g) 34
300
350
400
450
Fig.15 Variation of Vicker’s hardness number (Hv) with load (P) of UCHBS crystal
2.1
Log (d)
2.0
n=0.3173
1.9
1.8
1.7
1.4
1.6
1.8
2.0
Log (P)
35
2.2
2.4
2.6
Fig.16 Plot of log P vs. log d for UCHBS crystal
36
Fig.17 Etchpit patterns observed on UCHBS crystal
37
Table 1: Crystal structure data for UCHBS Parameters
Crystal data CH5N2O+·C7H5O6S−
Molecular formula Molecular weight
278.24 g/mol
Crystal system
Triclinic
Space group
P1
Hall symbol
-P 1
Unit cell dimensions
a = 7.219 (4) Å b = 8.748 (5) Å c = 10.139 (5) Å α = 85.504º β = 71.087º γ = 68.659º
Volume (V)
V = 566.50 Å3
Crystal size (mm)
0.30 × 0.24 × 0.20
38
Table 2: Assignment of infrared and FT-Raman vibrational frequencies of UCHBS crystal Wavenumber (cm-1) FT-IR
FT-Raman
Assignments
3406
-
Intermolecular O-H stretching
3223
-
N-H stretching
3136
3091
Aromatic C-H stretching
-
3036
carboxylic O-H stretching
2580
-
1665
1673
carbonyl C=O stretching
1631
1591
N-H bending
1480
-
1425
1410
C-N stretching
1352
1310
symmetric S=O stretching
1210
1235 & 1207
C-O stretching
1157
1174
symmetric S=O stretching
1080
1080
in plane C-H bending
-
1026
symmetric C-C stretching
934
883
S-O stretching
836
-
761
778
C-S stretching
-
720
C=O bending
662
-
589
586
O-H stretching
C-C ring stretching
out of plane C-H bending
in plane ring C-C bending out of plane ring C-C bending
39
Table 3: Z-scan experiment parameters observed for UCHBS crystal
Laser beam wavelength (λ)
632.8 nm
Lens focal length (f)
22.5 cm
Optical path distance (Z)
175 cm
Spot-size diameter in front of the aperture (ωa)
1cm
Aperture radius (ra)
4mm
Nonlinear refractive index (n2)
3.932 ×10−12m2/W
Nonlinear absorption coefficient (β)
2.653 ×10−4cm/W
Third-order nonlinear optical susceptibility (χ(3))
2.353×10−7 esu
40
Graphical abstract
41
Highlights
Single crystal of UCHBS size upto 16 x 12 x 4 mm3 was grown by solution growth method. FWHM traced on (110) plane of UCHBS crystal was found to be 116 arc sec. UV-Vis cut-off wavelength of UCHBS was found to be 345 nm. NLO parameters, n2 = 3.932×10−12 m2/W, and β = 2.653×10−4 cm/W were estimated.
42
S1386-1425(15)00114-6 http://dx.doi.org/10.1016/j.saa.2015.01.093 SAA 13261
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
11 October 2014 8 January 2015 29 January 2015
Please cite this article as: A. Silambarasan, M. Krishna Kumar, A. Thirunavukkarasu, I. MD Zahid, R. Mohan Kumar, P.R. Umarani, Studies on the growth, spectral, structural, electrical, optical and mechanical properties ofUronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third–order nonlinear optical applications, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa. 2015.01.093
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Studies on the growth, spectral, structural, electrical, optical and mechanical properties of Uronium 3-carboxy-4-hydroxybenzenesulfonate single crystal for third–order nonlinear optical applications A. Silambarasan a, M. Krishna Kumar a, A. Thirunavukkarasu a,b, I. MD Zahida, R. Mohan Kumar a,*, P.R. Umarani c a
Department of Physics, Presidency College, Chennai–600 005, India Department of Physics, S.I.V.E.T College, Chennai–600 073, India c Directorate of Collegiate Education, Govt. of Tamil Nadu, Chennai–600 006, India b
Abstract Organic Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) nonlinear optical single crystal was grown by solution growth technique. The solubility and nucleation studies were performed for UCHBS at different temperatures 30, 35, 40, 45, 50 and 55oC. The crystal structure of UCHBS was elucidated from single crystal X–ray diffraction study. High resolution X–ray diffraction technique was employed to study the perfection and internal defects of UCHBS crystal. Infrared and Raman spectra were recorded to analyze the vibrational behavior of chemical bonds and its functional groups. The physico-chemical changes, stability and decomposition stages of the UCHBS compound were established by TG–DTA studies. The dielectric phenomenon of UCHBS crystal was studied at different temperatures with respect to frequency. Linear optical properties of transmittance, cut-off wavelength, band gap of UCHBS were found from UV-visible spectral studies. Third–order nonlinear optical susceptibility, nonlinear refractive index, nonlinear optical absorption coefficient values were measured by Z–scan technique. The mechanical properties of UCHBS crystal was studied by using Vicker’s microhardness test. The growth features of UCHBS crystal were analyzed from etching studies. Keywords: Crystal growth; Infrared spectroscopy; High-resolution X-ray diffraction; Z-scan technique; 1
*Corresponding author: Dr. R. MOHAN KUMAR Department of Physics Presidency College Chennai-600 005, India Tel: +91-9444600670; Fax: +91-44-28510732 Email: [email protected]
1. Introduction Recently, materials with excellent nonlinear optical (NLO) properties and fast response have attracted much attention for their widespread applications in optical switching, signal processing, optical power limiting and optical communications, optical data storage applications [1–3]. Due to these challenging applications, the growth of optical single crystals is very important for the fabrication of technologically important devices [4–6]. The use of single crystals is clearly noticeable in electronics and optics [7]. The high NLO property is the needed quality for advanced hybrid materials useful in next generation optical technologies. It was found that organic materials show excellent nonlinearity than inorganics. The delocalized π-electron system enhances their asymmetric polarizability and it leads to the microscopic origin of nonlinearity in molecular organic NLO materials [8]. The novel molecular designs with higher third–order nonlinearity and to incorporate them into the devices are the urgent need for optical applications [9–10]. Urea based compounds offered a wide choice of materials and expected to play an important role in future [11]. The acid component 5–Sulfosalicylic acid is an excellent hydrogen donor and hydrogen acceptor and used to design novel crystal with Urea. 5–Sulfosalicylic acid has three potential coordination sites and it can give mono–, di– and tri–anionic ligand species through de-protonation. The presence of large amount of oxygen atoms in the functional groups can be afforded hydrogen bonded associations and used to self-assembly of crystallization [12]. 2
In the present report, Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) crystal was grown from aqueous solution by slow evaporation method. The grown UCHBS crystals were subjected to X-ray diffraction, infrared, Raman, thermal, dielectric, UV-visible spectral studies and mechanical studies. The third order nonlinear optical property of UCHBS crystal studied by employing Z–scan technique and results are presented.
2. Materials and methods The analytical grade starting materials Urea and 5–Sulfosalicylic acid were used for synthesis of UCHBS compound. BRUKER KAPPA APEX II X–ray diffractometer was used to determine the crystal system and lattice parameters of UCHBS crystal at 293 K. High resolution X–ray diffraction curve was recorded by using PANalytical X'PertPRO MRD system with MoKα1 radiation to assess the crystalline perfection. BRUKER IFS FT–IR and Bruker RFS27 spectrophotometers with 100 mW laser excitation were used to record Infrared (IR) and Raman spectrum respectively. TGA–DTA thermogram was traced from RT to 600oC using SDT Q600 V8 instrument. The dielectric measurements were carried out by using HIOKI 3532-50 LCR HITESTER in the frequency range 50 Hz–5 MHz. UV-visible spectrum was recorded for the grown crystal using VARIAN CARRY 5E photospectrometer. Z–scan technique was employed to determine third–order nonlinearity using He–Ne laser source at 632.8 nm. Vicker’s microhardness test was carried out for UCHBS crystal using Shimadzu HMV–2000 Vicker’s pyramidal indenter.
3
3. Experimental 3.1 Synthesis, Solubility, metastable zone width and Crystal Growth Uronium 3-carboxy-4-hydroxybenzenesulfonate (UCHBS) compound was synthesized by dissolving equimolar quantity of Urea (6.006 g) and 5-Sulfosalicylic acid (25.422 g) in 100 ml deionized water and stirred. The solution was magnetically stirred about 8 h to achieve homogeneous mixture and synthesized (Fig.1). It was dried at room temperature and synthesized salt was collected. Repeated recrystallization process was adopted for purification of UCHBS salt in aqueous solution and impurities were removed by filtration. The solubility and metastable zone width measurements were carried out in a constant temperature bath with an accuracy ± 0.01ºC. The solubility was determined by gravimetric analysis at different temperatures ranging 30–55ºC. 100 ml of deionized water (solvent) was taken in a flask and UCHBS salt was added upto the saturation point at 30oC. The solution was stirred for about 8 h to achieve homogenization by using an immersible magnetic stirrer and saturated solution was achieved. 10 ml of saturated solution was taken out and poured into the petri dish and dried. Then, the dried UCHBS salt was weighed. The same process was repeated for other temperatures. The solubility of UCHBS is shown in Fig.2. From the plot, it was observed that solubility curve shows a positive gradient which is suitable for slow cooling and evaporation crystal growth methods [13]. The metastable zone width experiment was carried out for UCHBS and metastable zone width was estimated by polythermal method [14]. It is a measure of nucleation temperature at a particular concentration of solute (Fig.2). For finding metastable zone width, newly prepared saturated solution at 30oC was slowly cooled until the first speck appeared and it was instantly recorded as the nucleation temperature. The difference between the saturation and nucleation temperature gives the metastable zone width of the 4
UCHBS in deionized water. The experiment was repeated at different saturation temperatures such as 30, 35, 40, 45, 50 and 55ºC. To grow the good UCHBS crystals, the purified UCHBS salt was dissolved in aqueous solution and saturated at 40oC. The saturated solution was kept at 40oC and maintained the temperature over the growth period. The supersaturated growth solution yielded good quality optical grade UCHBS single crystal in the period of three weeks and collected for characterization. The harvested as-grown UCHBS single crystal with dimension of 16 x 12 x 4 mm3 is shown in Fig.3.
4. Results and discussion 4.1 X–ray diffraction studies The single crystal X–ray diffraction data collection was made for UCHBS crystal at 293 K. It reveals that UCHBS crystal belongs to P1 space group of triclinic crystal system (Table.1). The observed unit cell values are a = 7.219 Å, b = 8.748 Å, c = 10.139 Å, α = 85.504º, β = 71.087º, γ = 68.659º and V = 566.50 Å3 [15]. Figure 4 shows the molecular structure of UCHBS unit molecule. The asymmetric unit of UCHBS consists of one CH5N2O+ cation and one C7H5O6S- anion. The dihedral angle between the benzene ring and the mean plane of the Urea fragment is 76.02 (8)°. In the anion, the carboxyl group makes the dihedral angle of 1.47 (9)° with the benzene ring. The molecular structure confirms that cation and anion are linked by O—H···O and N—H···O hydrogen bonds, and the molecular conformation of the anion is controlled by weak O—H···O hydrogen bond. The study of the defect distribution in the grown crystal is of significant importance because the defects created during growth have severe consequence on the physical properties of the crystal. The degree of crystalline perfection of the 5
UCHBS crystal was characterized by recording high–resolution X–ray diffraction (HRXRD) rocking curve using a multicrystal diffractometer (Fig.5). The absence of additional satellite peaks and the single diffraction curve (DC) peak show that, crystalline perfection of the grown UCHBS crystal is fairly good without having any internal structural grain boundaries. The FWHM (full width at half maximum) of the curve was found to be 116'' arc s which is acceptable for a perfect crystal [16].
4.2 Infrared and Raman spectral analysis The presence of functional groups in the UCHBS compound was confirmed by infrared (IR) and Raman spectral analysis and spectra are shown in Fig.6 and Fig.7 respectively. The assignments of bands observed in the vibration spectra are very useful to understand the chemical structure of UCHBS. The observed IR and Raman bands together with their functional assignments are presented in Table 2. The spectral analysis was performed on the basis of the characteristic vibrations of Urea and 5-sulfosalicylic acid presence in UCHBS compound. C–H Vibrations The C–H stretching vibration of hetero–aromatic structure is usually observed in the region 3200–3000 cm-1. Vibrational peaks at 3136 cm-1 in IR and 3091 cm-1 in Raman spectrum confirmed the presence of aromatic C–H stretching in UCHBS. The substitution sensitive C–H inplane bending vibrations lie in the region 1300–1000 cm-1[17]. The band observed at 1080 cm-1 is assigned to inplane C–H bending in IR and Raman spectra. Bands involving the C–H out of plane vibrations generally occur between 1000–675 cm-1[17] and it was observed at 836 cm-1 in IR spectrum. 6
O–H Vibrations Vibrations due to O–H stretching are sensitive. Hydrogen atom bonded with carboxylic groups display an intermolecular hydrogen bond formation and its vibrational frequency lies in the rage of 3590–2400 cm-1. It was observed at 3406 cm-1 in IR and 3036 cm-1 in Raman spectrum. The FT–IR band identified at 2580 cm-1 is assigned to O–H stretching vibration. The presence of carboxylic and water molecules in UCHBS was confirmed. N–H Vibrations N–H vibrations occur in the region of 3500–3200 cm-1 [18]. The band observed at 3223 cm-1 in IR spectrum is due to N–H stretching vibration. N–H bending vibrations were observed at 1631 cm-1 and 1591 cm-1 in IR and Raman spectra respectively. C–S Vibrations and C–N Vibrations The C–S stretching bands are usually observed in the range of 930–670 cm-1 [19,20] with medium intensity. It was observed at 761 cm-1 in FT-IR and 778 cm-1 in FT-Raman of UCHBS compound. C–N stretching vibrations were observed at 1425 cm-1 in IR spectrum and 1410 cm-1 in Raman spectrum. C=O Vibrations The absorptions are sensitive for both carbon and oxygen atoms of the carbonyl group in the vibrational spectrum. Both have the same amplitudes while it vibrates. Normally carbonyl group vibrations occur in the region 1870–1540 cm-1 [21]. In the present study, the peaks at 1665 cm-1 in IR and 1673 cm-1 in FT-Raman are due to carbonyl C=O stretching vibrations of UCHBS. The C=O bending vibration occurred at 720 cm-1 in the FT-Raman spectrum. 7
C–O Vibrations The C–O stretching vibration usually appears in the region 1300–1200 cm-1 [22]. In the present case, the vibration at 1210 cm-1 in IR and intense bands at 1235, 1207 cm-1 in Raman spectra were observed. C–C Vibrations The vibrational bands observed between 1650 and 1400 cm-1 in benzene derivatives are assigned to C–C ring stretching modes [23]. Accordingly in the present investigation, the carbon–carbon vibrations were observed at 1480 cm-1 in IR and the symmetric C–C stretching was noticed at 1026 cm-1 in FT-Raman spectra. The C–C in- plane vibration was found at 662 cm-1 in IR spectrum. The C–C out of plane and C–C stretching vibrational frequencies were found at 589 and 586 cm-1 in IR and FT-Raman spectra respectively. S=O and S–O Vibrations The sulfonate group vibrations S=O were occurred at 1352, 1157 cm-1 in IR and 1310, 1174 cm-1 in FT-Raman spectra. Also the vibrations found at 934 cm-1 and 883 cm-1 are assigned to S–O vibrations in IR and FT–Raman spectra respectively. 4.3 Thermal analysis Thermogravimetric analysis (TG-DTA) is the measure of the degradation of the compound with an increase of temperature. The experiment was carried out in nitrogen atmosphere with the heating rate of 10 oC/min. The resulting TG-DTA trace of UCHBS is shown in Fig.8. The first decomposition stage of UCHBS was observed between 194.82 oC and 236.49 oC and it was 8
about 6% of mass loss of the compound. Due to the decomposition, NH3 group (6% of mass) expelled from UCHBS. Similarly, a sharp endothermic peak observed at 194.67 oC in DTA curve reveals the melting point of UCHBS. It also indicated the good degree of crystallinity. During the first stage of decomposition, exothermic process was found at 221.82oC in DTA curve. The second decomposition stage was observed between 236.49oC and 302.54oC. It was due to the release of CO2 gas from Urea component of UCHBS and it was about 15 % weight loss from UCHBS. The third weight loss occurred between 302.54 oC and 343.70 oC is due to the release of NO2 gas and it caused about 12% weight loss of UCHBS. From the DTA curve, an exothermic peak observed at 339.76oC is due to the release of NO2 gas. The fourth weight loss occurred in the temperature range between 343.70 oC and 370.43 oC is due to the removal of CO2 gas by the decomposition of sulfosalicylic component and it was about 16% weight loss of UCHBS. The fifth decomposition stage was started at 370.43oC and went upto 461.12oC. It was noted due to the release of SO2 gas from sulfosalicylic component and it was about 23% of weight loss from UCHBS. The residue of UCHBS left at 600ºC was about 29%. From the TG curve, it is concluded that the UCHBS is stable upto 194.8oC. Also, it was observed that, there is no phase transition and weight loss before this temperature. The observed thermal stability of UCHBS is most useful for advanced optical applications. 4.4 Dielectric measurement The dielectric characteristics of the material are important to know the transport phenomena and the lattice dynamics of the crystal. It also gives the information about the nature of atoms, ions, bonding and their polarization mechanism of the material [24]. The crystals with high transparency and large surface defect free were selected and used for dielectric measurements. 9
The selected crystals were polished and silver paste was applied on both sides of the sample for good conduction. The dielectric studies were carried out for UCHBS in the temperature range 313–343 K and the frequency range of 50 Hz–5 MHz. The dielectric constant (εr) was calculated using the relation,
=
(1)
where C is the capacitance, d is the thickness of the crystal, εo is the permittivity of free space, and A is the area of the crystal used. The dielectric loss was calculated using the relation, ε'' = εrD
(2)
where D is the dissipation factor. The response of dielectric constant and dielectric loss as a function of frequency is shown in Fig.9&10. From the plots, it was observed that the dielectric constant and dielectric loss decreases with an increase in frequency and attains saturation at higher frequencies. The dielectric constant and dielectric loss measured at different temperatures almost remains the same at higher frequencies. It indicates that, the temperature variation has no influence on the dielectric property of the material. The low values of dielectric constant and dielectric loss at higher frequencies reveal the quality of grown UCHBS crystal with less defects, which is the desirable property of the materials to be used for various optical and communication applications [25]. Also, the low values of dielectric constant and dielectric loss at higher frequencies are important in the construction of photonic and NLO devices.
4.5 UV–Visible transmission studies Optical devices need crystals with highly transparent over a considerable region of wavelength and defect free [26]. UV–Vis spectrum gives useful information about the structure of the molecules that is the promotion of electrons in σ and π orbits. The thickness of the crystal 10
sample used for this measurement is 1.1 mm. The spectrum was recorded in the range 190-900 nm and shown in Fig.11. It shows good transmission window between 345 to 900 nm, which is most useful for visible region optical applications. The cut–off wavelength of UCHBS crystal was found to be 345 nm. UV-visible study indicates the good optical transmittance in the useful optical region of UCHBS crystal. 4.5.1 Optical band gap and refractive index estimation Optical band gap (Eg) and refractive index (no) of the material are important physical parameters for the optical crystals using in linear and nonlinear optical applications. The band gap energy of the material is very closely related to the material’s atomic and electronic band structures [27]. The energy gap of UCHBS crystal was calculated using the relation, =
λ
(3)
where h is the Planck’s constant, c is the velocity of light and λ is the cut-off wavelength of UCHBS crystal. The optical absorption coefficient (α) was calculated using the relation,
=
. (/)
(4)
where ‘T’ is the transmittance and ‘t’ is the thickness of the crystal. For the direct band gap of material, the absorption coefficient ( ) obeys the relation for high photon energies (ℎυ), ℎ = (ℎ − )/
(5)
where ‘A’ is a constant, ‘Eg’ is the optical band gap, ‘h’ is the Planck’s constant, and ‘υ’ is the frequency of the incident photons [27]. The band gap energy (Eg) was calculated from linear part of the Tauc’s plot drawn between (αhυ)2 and photon energy (hυ) (Fig.12). The band gap energy 11
of UCHBS crystal was found to be 3.46 eV. The refractive index of an optical material can be calculated using transmission spectrum and the reflectance of the crystal may be described in terms of absorption coefficient (α) [28], =1±
"#$%&(#')($%& (') #)*+ (#')
(6)
The refractive index (no) was calculated using the relation, , =
#(-()±√#-/ (-# (-#)
(7)
The calculated refractive index value (no) of UCHBS crystal is 2.63 and it can be used to calculate third-order nonlinear optical susceptibility of the material.
4.6 Z-scan measurement Z-scan technique is an effective and popular technique for characterizing the third-order optical nonlinearity of the materials [29, 30]. Z-scan measurement is a widely used to determine the magnitude and sign of the nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the samples. The measurements were performed in open aperture and closed aperture modes. The measurement in open aperture mode gives the information about the nonlinear absorption coefficient and closed aperture mode helps to calculate the nonlinear refractive index of the material (Fig.13 & 14). He-Ne laser with wavelength of 632.8 nm was used in this experiment. The beam radius of 1 mm was used to scan the sample. Optically polished UCHBS crystal of thickness 1 mm was used in this study. Experiment was performed by placing the sample in the path of the beam at different positions with respect to the focus and the respective light transmission was measured. 12
From the Z-scan data, the difference between the peak and valley transmittances (∆TP-V) can be calculated using the relation [31], ∆12#3 = 0.406(1 − 7).8 |∆φ |
(8)
where ∆φo is the axis phase shift at the focus, S is the linear transmittance aperture and it was calculated using the relation, /
7 = 1 − :;< =− ?/> @ >
(9)
where ra is the radius of aperture and ωa is the beam radius at the aperture. The nonlinear refractive index (n2) was calculated using the relation [32-34], , = BC
∆φA
(10)
A DEFF
where k is the wave number (k=2π/λ), Io is the intensity of the laser beam at the focus (Z=0) and Leff = [1-exp (-αL)]/α is the effective thickness of the sample, α is the linear absorption and L is the thickness of the sample. The nonlinear absorption coefficient (β) can be estimated from the open aperture Z-scan data.
G =
√.∆
(11)
CA DEFF
where, ∆T is peak value at the open aperture Z-scan curve. The value of β is negative for saturable absorption and positive for two photon absorption. The real and imaginary part of the third order nonlinear optical susceptibility (χ(3)) were estimated using [35], : χ() =
HI A J /KA/ K/ L
13
(MN ⁄O)
(12)
QN χ() =
H/A J / KA/λR SL/
(MN ⁄O)
(13)
where, εo is the vacuum permittivity, c is the velocity of light in vacuum. no is the linear refractive index of the sample, and λ is the wavelength of laser beam. The third order nonlinear optical susceptibility was calculated using the relation,
χ() = T( :χ() ) + (QNχ() )
(14)
The calculated third order nonlinear optical susceptibility is given in Table 3. Due to excellent nonlinear response, UCHBS can be a promising material for nonlinear optical device fabrication.
4.7 Microhardness and etching studies Microhardness test is one of the best methods of understanding the mechanical properties of materials such as hardness number, crack length, fracture toughness, brittle index and elastic stiffness constant. Hardness of a material is a measure of resistance its offers to local deformation [36]. In case of UCHBS crystal, loads ranging from 25 to 400 g were used for making indentations, keeping the time of indentation at 10 s for all the cases. The diagonal lengths of the indentation mark and the crack lengths were measured, using a micrometer eyepiece. The Vicker’s hardness value was calculated using, VW =
.X88S 2 Y/
(Z[/NN )
(15)
where, ‘Hv’ is the hardness number in kg/mm2, ‘P’ is the applied load in kg and ‘d’ is the average diagonal length of the indented impressions in ‘mm’. From the Figure 15, it was observed that the hardness value increases with increase in applied load, which is known as reverse indentation size effect (RISE) [37]. It clearly indicates that crystal possesses good mechanical strength. The relation connecting the applied load and diagonal length ‘d’ of the indenter is given by 14
Meyer’s law, P = adn, where ‘n’ is the Meyer’s index which is the measure of ISE and RISE and ‘a’ is the constant. For the normal ISE behavior, the exponent n value is < 2 and also n > 2 for the RISE behavior. When n = 2, the hardness is independent of the applied load, and it is given by Kick’s law. The ‘n’ value of UCHBS crystal was obtained by using linear fit (Fig.16) and it was found to be 0.3173. When the indenter just touches the surface, there is an increase in the hardness value with load, which can be attributed to the work hardening of the surface layers [38-39]. The microhardness value attains nearly a constant due to the rearrangement of dislocations and mutual interaction of dislocations. The result of Vicker’s microhardness test confirms that, UCHBS is classified as soft category material.
The crystallization perfection and growth features can be analyzed from etching studies. The etch patterns on the flat surface of UCHBS crystal were analyzed using water etchant and it was observed by using a high resolution microscope. The rectangular etch pits was observed on the surface of UCHBS crystal as shown in Fig.17. The sharp rectangular etch pits were noticed on the grown crystal for etching time of 10 s and further increase of etching time (15 s), broad rectangular etch pits were observed on the surface of the crystal. For the etching time of >20 s, etch pits were disappeared. This shows the good crystalline nature as well as the regular growth pattern of the UCHBS crystal.
5. Conclusion Optical quality single crystal of UCHBS was grown by slow evaporation method. The solubility and metastable zone width were determined for UCHBS salt in deionized water. The crystal structure and crystalline perfection of UCHBS were analyzed from single crystal and HRXRD studies. The chemical composition of UCHBS was analyzed by FT-IR and 15
FT-Raman spectral studies. Thermal stability of UCHBS compound was estimated using TG-DTA analyses. Low value of dielectric constant and dielectric loss for various temperatures confirmed the quality of the UCHBS crystal. UV–visible studies confirm that grown crystal can be useful for optical applications in the wavelength region 345–900 nm. The optical band gap energy, nonlinear optical refractive index (n2), nonlinear absorption coefficient (β) and thirdorder nonlinear optical susceptibility (χ(3)) were calculated by Z-scan technique. Vicker’s microhardness test reveals that UCHBS belongs to soft category NLO crystal. The etching study revealed the growth mechanism of UCHBS crystal, which found to have layered growth with rectangular etch pits.
16
References [1] Q. Chen, E. Sargent, N. Leclerc, A. Attias, Phys. Lett. 82 (2003) 4420–4422. [2] G. Stegeman, E. Wright, N. Finlayson, R. Seaton, C. Seaton, J. Lightwave Technol. 6 (1988) 953–970. [3] A. Slepkov, F. Hegmann, Y. Zhao, R. Tykwinski, K. Kamada, J. Chem. Phys. 116 (2002) 3834–3840. [4] L. Kuang, Q. Chen, E. Sargent, Z.J. Wang, Am. Chem. Soc. 125 (2003) 13648. [5] M. Krishna Kumar, S. Sudhahar, A. Silambarasan, B.M. Sornamurthy, R. Mohan Kumar Optik 125 (2014) 751–755. [6] A.M. Petrosyan, J. Crystallization Phys. Chem. 1 (2010) 33–56. [7] Mohd. Shkir, B. Riscob, V. Ganesh, N. Vijayan, Rahul Gupta, J.L. Plaza, E. Dieguez, G. Bhagavannarayana J. Cryst. Growth 380 (2013) 228–235. [8] D. Balasubramanian, R. Sankar, V. Siva Sankar, P. Murugakoothan, P. Arulmozhichelvan, R. Jayavel, Mater. Chem. Phys. 107 (2008) 57–60. [9] M. Somac, A. Somac, B.L. Davies, M.G. Humphery, M.S. Wong, Opt. Mater. 21 (2002) 485–488. [10] L.V. Natarajan, R.L. Sutherland, V.P. Tondiaglia, T.J. Bunning, W.W. Adams, J. Nonlinear Opt. Phys. Mater. 5 (1996) 89–98. [11] T. UmaDevi, N. Lawrence, R. RameshBabu, S. Selvanayagam H. StoeckliEvans, K. Ramamurthi, J. Cryst. Growth 311 (2009) 3485–3490. [12] L. Wang, L.Y. Xu, R.F. Xue, X.F.Lu, R.X. Chen and X.T. Tao, Sci. China Chem. 55 (2012) 138–144. [13] A. Deepthy, HL. Bhat, J. Cryst. Growth 226 (2001) 287–293. 17
[14] K. Sangwal, J. Cryst. Growth 97 (1989) 393−405. [15] A. Silambarasan, M. Krishna Kumar, G. Chakkaravarthi, R. Mohan Kumar, P.R. Umarani Acta Cryst. E 69 (2013) o1725. [16] B.W. Batterman, H Cole, Rev. Mod. Phys. 36 (1964) 681–717. [17] M. Govindarajan, S. Periandy, K. Carthigayen, Spectrochim. Acta 97 (2012) 411–422. [18] A. Usha Rani, N. Sundaraganesan, M. Kurt, M. Cinar, M. Karabacak, Spectrochim. Acta Part A 75 (2010) 1523–1529. [19] N.B. Colthup, L.H. Daly, S.E. Wiberly, Introduction to Infrared and Raman Spectroscopy, Third Ed., Acadamic Press, Boston, MA, 1990. [20] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley and Sons, New York, 1980. [21] D.N. Sathyanarayana, Vibrational Spectroscopy, Theory and Applications, New Age International (P) Limited Publishers, New Delhi, 1996. [22] A. Silambarasan, M. Krishna Kumar, A. Thirunavukkarasu, R. Mohan Kumar, P.R. Umarani, Spectrochim. Acta Part A 135 (2015) 39–45. [23] S. Mohan, N. Sundara Ganesan, J. Mink, Spectrochim. Acta A 47 (1991) 1111–1115. [24] P. Krishnan, K. Gayathri, G. Bhagavannarayana, S. Gunasekaran, G. Anbalagan Spectrochim. Acta Part A. 102 (2013) 379–385. [25] C. Balarew, R. Duhlew, J. Solid State Chem. 55 (1984) 1–6. [26] V. Krishnakumar, R. Nagalakshmi, Spectrochim. Acta A 61 (2005) 499–507. [27] R.N. Rai, S.R. Mudunuri, R.S.B. Reddi, V.S.A. Kumar Satuluri, S. Ganeshmoorthy, P.K. Gupta, J. Cryst. Growth 321 (2011) 72–77. [28] R. Robert, C. Justin Raj, S. Krishnan, S. Jerome Das, Physica B 405 (2010) 20–24. 18
[29] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, Opt. Lett. 14 (1989) 955–957. [30] M. Sheik-Bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760–769. [31] T. Kanagasekaran, P. Mythili, P. Srinivasan, A.Y. Nooraldeen, P.K. Palanisamy, R. Gopalakrishanan, Cryst. Growth Des. 8 (2008) 2335–2339. [32] G. AnandhaBabu, P. Ramasamy, Spectrochim. Acta Part A 82 (2011)521–526. [33] A. Subashini, R. Kumaravel, S. Leela, H.S. Evans, D.Sastikumar, K. Ramamurthi, Spectrochim. Acta Part A 78 (2011) 935–941. [34] R. Ashok Kumar, R. Ezhilvizhi, N. Vijayan, G. Bhagavannarayana, D. Rajanbabu, J. Pure Appl. and Indus. Phys. 1 (2010) 61–67. [35] D. Bharath, S. Kalainathan, Spectrochim. Acta Part A 118 (2014) 1098–1105. [36] W. Mott, Micro Indentation Hardness Testing, Butterworths, London, 1956. [37] K. Sangwal, B. Surowska, P. Błaziak, Mater. Chem. Phys. 77 (2002) 511–520. [38] E. M. Onitsch, Mikroskopie 2 (1947) 131–151. [39] P. Vivek, P. Murugakoothan, Optic. Laser Tech. 49 (2013) 288–295.
19
Fig.1 Material synthesis scheme for UCHBS
20
55
Solubility curve Nucleation curve
45
o
Temperature ( C)
50
40 35
M
n zo e l tab s a et
id ew
th
30 25 20 70
80
90
100
110
120
130
140
Concentration (g/100 ml) Fig.2 Solubility and nucleation curves of UCHBS in water solvent
21
Fig.3 Photograph of UCHBS single crystal grown by slow evaporation method
22
Fig.4 The molecular structure of UCHBS unit molecule
23
140000
ΜοΚα1
-1 Diffracted X-ray Intensity (cs )
36" 120000
(110) plane (+,-,-,+)
100000 80000
116"
60000 40000 20000 0 -400
-200
0
200
400
Glancing angle (arcsec) Fig.5 High-resolution X-ray diffraction curve recorded for a typical UCHBS single crystal specimen using (110) diffracting plane
24
1.2
1942
0.8
4000
3500
3000
2500
2000
1500 -1
Wavenumber (cm )
Fig.6 FT-IR spectrum of UCHBS crystal
25
532
662
1000
589
3223 3136
++
1210 1157 1080
1665 1631
2580
0.2
0.0
1480 1425 1352
0.4
934 836 761
0.6
3406
Transmittance (%)
1.0
500
73
0.8
1207 1174
0.7
1235
0.4
1673 1591 1410 1310
0.3 0.2 0.1
1026 883 778 720 586 527 444 251 170
1080
0.5
3091 3036
Raman Intensity
0.6
0.0 4000
3500
3000
2500
2000
1500
1000
Wavelength (nm)
Fig.7 FT-Raman spectrum of UCHBS crystal
26
500
0
o
+ +
80
+
Weight %
Exo
o
+ 302.54
10
TGA DTA
+
221.82 C
70
+
o
C
5
343.70 oC
60
+
0 50
+ 370.43
o
C
40
-5
o
461.12 C
+
30
-10
20 100
200
300 400 o Temperature C
500
Fig.8 TG-DTA curve of UCHBS crystal
27
600
Temperature difference oC/mg
o C o C 4 9 5 4 1. 6. 29 23
o
194.67 C
90
C C
+194.82
33 9. 76
100
15
Endo
o
300
313 K 323 K 333 K 343 K
280 260
Dielectric constant
240 220 200 180 160 140 120 100 80 60 1
2
3
4
5
6
Log f Fig.9 Plot of Dielectric constant vs. Log f of UCHBS crystal
28
7
8
313 K 323 K 333 K 343 K
7
Dielectric Loss
6 5 4 3 2 1 0 1
2
3
4
5
6
Log f Fig.10 Plot of Dielectric loss vs. Log f of UCHBS crystal
29
7
100
Transmittance (%)
80
60
40
20
0
100
200
300
400
500
600
700
800
900
Wavelength (nm)
Fig.11 UV-Visible transmission spectrum of UCHBS crystal
30
0.06
(αhν)
2
(eV/m)
2
0.05 0.04 0.03 0.02 0.01 0.00 Eg= 3.46 eV
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy (eV) Fig.12 Tauc’s plot drawn between (αhυ)2 and photon energy (hυ) for UCHBS crystal 31
0.80
Normalized Transmittance
0.75 0.70 0.65 0.60 0.55 0.50 0.45 -8
-6
-4
-2
0
Z (mm)
32
2
4
6
8
Fig.13 Z-scan plot of UCHBS crystal in open aperture
Normalized Transmittance
0.81
0.80
0.79
0.78
-8
-6
-4
-2
0
Z (mm) 33
2
4
6
8
Fig.14 Z-scan plot of UCHBS crystal in closed aperture
50
2
Hardness number H V (kg/mm )
45 40 35 30 25 20 15 0
50
100
150
200
250
Load P (g) 34
300
350
400
450
Fig.15 Variation of Vicker’s hardness number (Hv) with load (P) of UCHBS crystal
2.1
Log (d)
2.0
n=0.3173
1.9
1.8
1.7
1.4
1.6
1.8
2.0
Log (P)
35
2.2
2.4
2.6
Fig.16 Plot of log P vs. log d for UCHBS crystal
36
Fig.17 Etchpit patterns observed on UCHBS crystal
37
Table 1: Crystal structure data for UCHBS Parameters
Crystal data CH5N2O+·C7H5O6S−
Molecular formula Molecular weight
278.24 g/mol
Crystal system
Triclinic
Space group
P1
Hall symbol
-P 1
Unit cell dimensions
a = 7.219 (4) Å b = 8.748 (5) Å c = 10.139 (5) Å α = 85.504º β = 71.087º γ = 68.659º
Volume (V)
V = 566.50 Å3
Crystal size (mm)
0.30 × 0.24 × 0.20
38
Table 2: Assignment of infrared and FT-Raman vibrational frequencies of UCHBS crystal Wavenumber (cm-1) FT-IR
FT-Raman
Assignments
3406
-
Intermolecular O-H stretching
3223
-
N-H stretching
3136
3091
Aromatic C-H stretching
-
3036
carboxylic O-H stretching
2580
-
1665
1673
carbonyl C=O stretching
1631
1591
N-H bending
1480
-
1425
1410
C-N stretching
1352
1310
symmetric S=O stretching
1210
1235 & 1207
C-O stretching
1157
1174
symmetric S=O stretching
1080
1080
in plane C-H bending
-
1026
symmetric C-C stretching
934
883
S-O stretching
836
-
761
778
C-S stretching
-
720
C=O bending
662
-
589
586
O-H stretching
C-C ring stretching
out of plane C-H bending
in plane ring C-C bending out of plane ring C-C bending
39
Table 3: Z-scan experiment parameters observed for UCHBS crystal
Laser beam wavelength (λ)
632.8 nm
Lens focal length (f)
22.5 cm
Optical path distance (Z)
175 cm
Spot-size diameter in front of the aperture (ωa)
1cm
Aperture radius (ra)
4mm
Nonlinear refractive index (n2)
3.932 ×10−12m2/W
Nonlinear absorption coefficient (β)
2.653 ×10−4cm/W
Third-order nonlinear optical susceptibility (χ(3))
2.353×10−7 esu
40
Graphical abstract
41
Highlights
Single crystal of UCHBS size upto 16 x 12 x 4 mm3 was grown by solution growth method. FWHM traced on (110) plane of UCHBS crystal was found to be 116 arc sec. UV-Vis cut-off wavelength of UCHBS was found to be 345 nm. NLO parameters, n2 = 3.932×10−12 m2/W, and β = 2.653×10−4 cm/W were estimated.
42