Studies on the optical and mechanical properties of non-linear optical 3-aminophenol orthophosphoric acid (3-amphph) single crystal

Optik 123 (2012) 1082–1086

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Studies on the optical and mechanical properties of non-linear optical 3-aminophenol orthophosphoric acid (3-amphph) single crystal K. Russel Raj, P. Murugakoothan ∗ PG & Research Department of Physics, Pachaiyappa’s College, Chennai 600030, India

a r t i c l e

i n f o

Article history: Received 6 January 2011 Accepted 2 May 2011 Keywords: Single crystal growth Optical properties Band gap energy Refractive index Hardness

a b s t r a c t Single crystals of semiorganic material 3-aminophenol orthophosphoric acid (denoted as 3-amphph) of size 29 × 17 × 4 mm3 have been grown by the slow evaporation of an aqueous solution of deionized water at 50 ◦ C. The crystal belongs to orthorhombic system with the non centrosymmetric space group P21 21 21 . ˚ b = 9.782(4) A˚ and c = 18.326(4) A. ˚ The The lattice parameter values of 3-amphph crystal are a = 4.481(2) A, grown crystals are subjected to single crystal XRD studies to identify its morphology and structure. Optical transmittance and second harmonic generation of the grown crystals have been studied by UV–Vis–NIR spectrum and Kurtz powder technique respectively. The transmittance of 3-amphph crystal has been used to calculate the refractive index n, the extinction coefficient k, reflectance R and both the real (εr ) and imaginary (εi ) components of the dielectric constant as a function of wavelength. The optical band gap of 3-amphph is 4.05 eV with direct transition. The anisotropic mechanical behavior of 3-amphph has been analyzed using Vickers microhardness test. The mechanism of growth is revealed by carrying out chemical etching using water as etchant. © 2011 Published by Elsevier GmbH.

1. Introduction Nonlinear optical materials find a variety of applications to perform functions like frequency conversion, light modulation, optical switching, optical memory storage and optical second harmonic generation (SHG) [1,2]. These applications depend on the various properties of the materials, such as transparency, birefringence, refractive index, dielectric constant and thermal, photochemical and chemical stabilities [3]. A strong need continues to exist for low cost, more efficient, high average power materials for optical parametric amplifier operation through the blue near-UV spectral region [4]. Since most of the organic crystals are constituted by weak van der Waals and hydrogen bonds with conjugated ␲ electrons they are soft in nature and difficult to polish. Also these materials exhibit intense absorption in UV region [5,6]. To overcome these problems, the research of combination of organic and inorganic hybrid compounds leads to find a new class of materials, called semiorganic materials for electronic industries. In semiorganic crystals, high optical nonlinearity of a purely organic ion is combined with the favorable mechanical and thermal properties of an inorganic counter ion [7,8]. The advantages of semiorganic

materials include they can be grown from aqueous solution and form large tree-dimensional crystals. A molecular crystal is built from an inorganic acid and an organic base, in which acid part of the molecular complex thus created, is responsible for favorable chemical, mechanical and thermal properties, due to strong hydrogen bond interaction which stabilize the crystal lattice [9]. The organic part, due to its relatively high hyperpolarizability value ˇ, is mainly responsible for the non linear optical properties of the crystal. It is interesting to note that phosphoric acid relatively easily forms molecular crystals with different organic bases which exhibit non linear optical properties [10]. One such effort has been made in the case of orthophosphoric acid with 3-aminophenol by Glowiak et al. [11]. In the present investigation, the bulk single crystal of 3-amphph has been grown by submerged seed solution evaporation method. The grown crystals are subjected to single crystal XRD, liner and nonlinear optical mechanical studies and etching analysis. The results are discussed in detail. 2. Experimental 2.1. Synthesis and crystal growth

∗ Corresponding author. Tel.: +91 044 26507586; mobile: +91 944 444 7586. E-mail address: [email protected] (P. Murugakoothan). 0030-4026/$ – see front matter © 2011 Published by Elsevier GmbH. doi:10.1016/j.ijleo.2011.07.036

The starting material was synthesized by taking 3-aminophenol (Loba Chemie – AR grade) and orthophosphoric acid (Merck) in a 1:1 stoichiometric ratio. The required amount of starting materials

K.R. Raj, P. Murugakoothan / Optik 123 (2012) 1082–1086

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Fig. 1. As-grown single crystal of 3-amphph.

for the synthesis of 3-amphph salt is calculated according to the following reaction: C6 H7 NO + H3 PO4 → C6 H10 NO5 P The calculated amount of orthophosphoric acid is first dissolved in deionized water and then 3-aminophenol is added to the solution slowly by stirring. The prepared solution is first heated to 50 ◦ C and then allowed to dry at room temperature and then the salt is obtained by slow evaporation technique. The purity of the synthesized salt is further improved by successive recrystallization process. Bulk crystals are grown from the saturated solution of 3-amphph, in a crystallizer using submerged seed solution evaporation method. Crystals of size 29 × 17 × 4 mm3 are obtained in a period of 20 days and the grown crystals are shown in Fig. 1. The grown crystals of 3-amphph are with well-defined growth faces and are nonhygroscopic. The morphology and crystallographic faces of the crystal studied using single crystal XRD are shown in Fig. 2. 3. Characterizations In order to determine the crystal structure and morphology of 3amphph crystals, single crystal X-ray diffraction studies have been carried out using Enraf Nonius CAD4 diffractometer with Mo K␣ ˚ The optical absorption spectra for 3-amphph single ( = 0.7170 A). crystal have been recorded in the region 200–1100 nm using Varian Carry SE model spectrometer to study their transmission behavior to electromagnetic radiation. The second harmonic generation (SHG) test on the 3-amphph crystal has been performed by the Kurtz and Perry powder SHG method [12]. The microhardness of the 3-amphph crystal has been measured using the Reichert MD 4000E ultra microhardness tester fitted with a Vickers diamond pyramidal indenter attached to a Reichert Polyvar 2 MET microscope. 4. Results and discussion

determined unit cell parameters are presented in Table 1 in comparison with those reported [11] and it shows that they are in close agreement. Kurtz and Perry powder SHG method [12] has been performed to determine the NLO property of 3-amphph crystal. The result obtained for 3-amphph shows a powder SHG efficiency of about 2.22 times that of KDP crystal. The SHG efficiency of 3-amphph is good in comparison with some of the semiorganic crystals [13,14], shown in Table 2. 4.2. Optical studies The optical transmission range, transparency cut-off and absorbance band are the most important optical parameters for laser frequency conversion applications. To find the transmission range of 3-amphph, the optical transmission spectrum is observed for the wavelength between 200 and 1100 nm (Fig. 3a). A crystal thickness 2 mm is used for this measurement. 3-Amphph is optically transparent in the entire visible region with 90% transmittance level and lower cut-off wavelength 306 nm which is sufficient for SHG laser radiation of 1064 nm or other applications in the blue region. The measured transmittance (T) is used to calculate the absorption coefficient (˛) using the formula ˛=

2.303 log(1/T ) t

where t is the thickness of the sample. The optical band gap (Eg ) is evaluated from the transmission spectra and the optical absorption coefficient (˛) near the absorption edge is given by [15] ˛h = A(h − Eg )

1/2

where A is a constant, Eg the optical band gap, h the Plank’s constant and  the frequency of the incident photons. The Tauc’s graph [16] plotted between the product of absorption coefficient and the incident photon energy (˛h)2 with the photon energy (h) at room

4.1. Single crystal X-ray diffraction and SHG efficiency From the single crystal X-ray diffraction analysis, the lattice parameters of 3-amphph crystal have been calculated by least square refinement of 25 reflections in the range of 20–30◦ . The

Table 1 Lattices parameter values for 3-amphph. Single crystal XRDReported [11]

Single crystal XRDPresent work

a = 4.493(2) A˚ b = 9.750(3) A˚ c = 18.328(4) A˚ V = 802.890 A˚ 3

a = 4.481(2) A˚ b = 9.782(4) A˚ c = 18.326(4) A˚ V = 803.331(5) A˚ 3

Table 2 Comparison of SHG efficiencies of promising semiorganic NLO crystals to potassium dihydrogen phosphate (KDP) equaling 1.0.

Fig. 2. Morphology of 3-amphph.

Nonlinear optical crystals

SHG efficiency

3-Amphph (present study) l-Alanine acetate l-Arginine chloride l-Arginine bromide l-Arginine tetrafluroborate l-Arginine diphosphate l-Histidine bromide [14]

2.22 0.30 0.20 0.30 0.54 0.98 1.00

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0.8

60

0.6

40

303 nm

0.4

20 0.2 (b)

0

400

600

800

1000

10

(a)

8 6 4

μ = 1.66

2 0 400

% Reflectance (R)

80

Extinction coefficient (k)

% Transmittance

(a)

Refractive index (n)

1.0

100

0.0 1200

200

(b)

150

1000

1200

1000

1200

50 0 400

600

800

Wavelength (nm)

Fig. 3. Plot of (a) wavelength vs % transmittance and (b) wavelength vs extinction coefficient.

25

Fig. 5. Plot of (a) wavelength vs refractive index (n) and (b) wavelength vs reflectance (R).

The reflectance (R) in terms of the absorption coefficient can be obtained from the equation [24]

-2

20

R=

15

exp(−˛t) ±



exp(−˛t)T − exp(−3˛t)T + exp(−2˛t)T 2 exp(−˛T ) + exp(−2˛t)T

2

2

800

Wavelength (nm)

100

Wavelength (nm)

(α h ν) eV cm

600

The refractive index (n) can be determined from reflectance data using the following relation [23] √ −(R + 1) ± 2 R n= R−1

10

5

Eg = 4.05 eV

0 1

2

3

4

5

6

7

(hν) eV

Fig. 4. Plot of (˛h)2 vs photon energy (h).

temperature shows a linear behavior that can be considered as evidence of the direct transition. The optical band gap (Eg ) of 3amphph crystal has been estimated by extrapolation of the linear portion near the onset of absorption edge to the energy axis [17]. From Fig. 4, value of optical band gap energy of 3-amphph crystal is found to be 4.05 eV and is direct transition in nature. The observed value is greater than that of the other NLO materials [18–21] shown in Table 3. The optical properties of the crystals are governed by the interaction between the crystal and the electric and magnetic fields of the electromagnetic waves. Extinction coefficient is the fraction of light lost due to scattering and absorption per unit distance in a participating medium. In electromagnetic terms, the extinction coefficient can be explained as the decay or damping of the amplitude of the incident electric and magnetic fields. The extinction coefficient (k) shown in Fig. 3b can be obtained from the equation:

K=

˛ 4

εr = ε0 + 4c = n2 − k2 Hence c =

n2 − k2 − ε0 4

where ε0 is the dielectric constant in the absence of any contribution from free carriers. The value of electrical susceptibility is 0.2209 at  = 1100 nm. The real part of dielectric constant εr and imaginary part of dielectric constant εi can be calculated following the relation [25] εr = n2 − k2

and

εi = 2nk

The values of real εr and imaginary εi dielectric constant, at  = 1100 nm are 2.755 and 2.62 × 10−4 respectively. 4.3. Mechanical analysis

The transmittance (T) is given by the following relation [22] T=

The wavelength dependence of n for 3-amphph crystal and reflectance (R) in the range 250–1100 nm are shown in Fig. 5a and b. Initially the refractive index decreases with increasing wavelength, then becomes constant. The refractive index n for 3-amphph crystal is 1.66 in the range 350–1100 nm. From the optical constants, the electric susceptibility (c ) can be calculated using the following relation [24]

(1 − R)2 exp(−˛t) 1 − R2 exp−˛t

Table 3 Comparison of band gap energy (eV). Materials

Band gap energy (eV)

3-Amphph (present study) 2-Aminopyridinium 4-nitrophenolate 4-nitrophenol [18] Benzophenone hydrazone [19] Barium strontium borate [20] Tyrosine hydro bromide [21]

4.05 2.50 3.25 2.30 3.02

Hardness of the material is a measure of resistance it offers to the local deformation [26]. The structure and composition of the crystalline solids are inviolably related to the mechanical hardness. Microhardness testing is one of the best methods of understanding the mechanical properties of the materials such as fracture behavior, yield strength, brittleness index and temperature of cracking [27,28]. The hardness measurements for 3-amphph crystal have been carried out on the prominent (0 0 1) and (0 1 1) planes of the crystal of thickness 5 mm using Reichert Polyvar 2 MET microscope. Loads ranging from 0.5 to 5 g are used for making indentations, keeping the time of indentation constant at 3 s for all cases. The microhardness value is calculated using the following relation Hv =

1.8544P N/m2 d2

K.R. Raj, P. Murugakoothan / Optik 123 (2012) 1082–1086 10000

200

14

Stiffness constant C11 (x10

2

160 (011) 140 120 100 80

(001) plane (011) plane

Pa)

(a)

180

Hardness number (kg/mm )

1085

(001)

60

(b)

40

8000

6000

4000

2000

0

20 0 0

20

40

60

80

100

0

120

20

40

80

100

120

Fig. 8. Plot of load P vs stiffness constant.

Fig. 6. (a) Vickers hardness vs load for 3-amphph (0 0 1) plane and (b) (0 1 1) plane.

where Hv is the Vickers hardness number, P is the applied load and d is the average diagonal length of the indentation mark. Measurement of hardness is a useful non destructive testing method used to determine the applicability of the crystal in the device fabrication. Indentations are made on the (0 0 1) and (0 1 1) faces of the crystal and the microhardness measurements have been made for the applied loads varying from 1 to 120 g for the dwell time 3 s. A plot between the hardness number and the load is depicted in Fig. 6. We clearly infer that the microhardness number increases with increasing load. A plot obtained between log(P) and log(d), shown in Fig. 7, gives a straight line. The relation connecting the applied load and diagonal length d of the indenter is given by Meyer’s law P = adn [29], where the exponent n called as the Meyer number, is the measure of the ISE and a is the constant. The simplest way to describe the ISE is Meyer’s law. For the normal ISE behavior, the exponent n < 2. When n > 2, there is the reverse ISE behavior. When n = 2, the hardness is independent of the applied test load, and is given by Kick’s law [30,31]. The value of n obtained for 3-amphph crystal in the planes (0 1 1) and (0 0 1) using linear fit is found to be 2.23 and 2.01 respectively. It is evident from the above plot that the microhardness value of the crystal increases with increase in load which is in agreement with the reverse indentation size effect (ISE) [32]. From careful observations on various materials, Onitsch [33] and Hanneman pointed out that n lies between 1 and 1.6 for hard materials, and it is more than 1.6 for soft materials. The value of n for the planes (0 1 1) and (0 0 1) obtained for 3-amphph crystals is 2.23 and 2.01 which reveals that the material is soft. From the hardness value, the yield strength  y can be calculated [34]. The yield strength ( y ) for planes (0 1 1) and (0 0 1) is found to be 59.91 MPa and 39.54 MPa respectively. The elastic stiffness constant (C11 ) for different loads (Fig. 8) calculated using Wooster’s [35] empirical formula C11 = Hv7/4 is depicted in Table 4 which gives an idea about tightness of bonding between neighboring atoms.

60

Load (g)

Load P (g)

Table 4 The values of elastic stiffness constants for different loads for 3-amphph. Loads (g)

C11 (×1014 Pa) for (0 0 1) plane

C11 (×1014 Pa) for (0 1 1) plane

10 20 30 40 50 60 70 80 82.5 85 87.5 90 92.5 100 110 120

107.7442 332.3463 653.0111 1215.311 1342.938 1956.247 2187.000 2880.147 3184.447 3392.648 3577.252 3682.929 3559.767 2998.131 2528.394 976.6446

11.31371 54.28072 182.5779 382.3179 630.6601 1248.563 2139.969 3279.969 4388.921 5312.073 7261.069 8691.375 8504.468 5570.435 4844.385 3760.07

4.4. Chemical etching analysis Chemical etching is a simple and very powerful tool to analyze the defects present in the growing crystal surfaces. Dislocations easily appear in crystals, especially in the initial stages of their growth. The chemical etching studies have been carried out on the as grown single crystals of 3-amphph to study the growth history and the distribution of the structural defects in the grown crystals [36,37]. Fig. 9 shows the surface of the as-grown single crystal etched for 15 s. The etching was carried out using methanol as the etchant at room temperature. Once the damaged surface layer was removed by means of etching, a fresh surface appeared which in turn gave clear etch pits. The etched surface were dried by gently pressing them between two filter papers and then immediately examined and their microstructure was analyzed using an

1.7 1.6

(011) Linear Fit

1.5

(a)

1.4 n = 2.23

1.3 1.2 1.1 1.0 0.9 1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

Log d

1.7 1.6

(001) Linear Fit

1.5 1.4

(b)

1.3 1.2

n = 2.01

1.1 1.0 0.9 1.35

1.40

1.45

1.50

1.55

1.60

1.65

1.70

Log P (g)

Fig. 7. Plot of log d vs log P.

Fig. 9. Etching micrograph on (1 0 0) plane for 15 s.

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optical microscope (Leitz Metallux-II) in the reflection mode. Some well defined and crystallographically aligned rectangular etch pits were observed on the as-grown surface. The etch pit density for (0 0 1) plane is found to be 2.2 × 103 cm−2 which is comparable to the reported value, sulphamic acid, SR-grown TGS and KAP [38]. 5. Conclusion Optical quality single crystals of semiorganic 3-amphph have been grown using solution growth technique. The grown crystals are observed to be transparent and colorless with well defined prismatic morphology. The unit cell parameter values show that the 3-amphph single crystal belongs to orthorhombic system with non centrosymmetric space group P21 21 21 . Optical studies show that the crystal has wide transmission range with UV cut-off wavelength at 303 nm. The optical band gap (Eg ), absorption coefficient (˛), extinction coefficient (k), refractive index (n), reflectance (R) have been calculated as a function of wavelength. The SHG efficiency of the grown crystal in frequency conversion is found to be 2.22 times that of KDP. Hardness measurement shows that 3amphph crystal is mechanically stable up to 110 g and increase of the work hardening is may be due to the dislocation motion. The etch pit density for 3-amphph is calculated using chemical etching analysis. References [1] W.S. Wang, M.D. Aggarwal, J. Choi, T. Gebre, A.D. Shields, B.G. Penn, D.O. Frazier, Solvent effects and polymorphic transformation of organic nonlinear optical crystal l-pyroglutamic acid in solution growth processes. I. Solvent effects and growth morphology, J. Cryst. Growth 198/199 (1999) 578–582. [2] S. Chenthamari, D. Jayaraman, P.M. Ushasree, K. Meera, C. Subramanian, P. Ramasamy, Experimental determination of induction period and interfacial energies of pure and nitro doped 4-Hydroxyacetophenone single crystals, Mater. Chem. Phys. 64 (2000) 179–183. [3] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Organic Molecules and Polymer, John Wiley & Sons Inc., New York, USA, 1991. [4] D.S. Chemla, J. Zyss, in: P.F. Liao, P. Kelley (Eds.), Quantum Electronics (Principles and Applications), Academic Press, New York, 1987. [5] C.B. Aakeroy, P.B. Hitchcock, B.D. Moyle, K.R. Seddon, Crystal engineering: hydrogen-bonded salts of hydroxybenzoic acids for second harmonic generation, J. Chem. Soc. Chem. Commun. (1993) 152–156. [6] R. Masse, Nonlinear Opt. 9 (1995) 113–126. [7] S. Debrus, H. Ratajczak, J. Venturini, N. Pincon, J. Baran, J. Barycki, T. Glowiak, A. Pietraszko, Novel nonlinear optical crystals of noncentrosymmetric structure based on hydrogen bonds interactions between organic and inorganic molecules, Synth. Met. 127 (2002) 99–104. [8] R. Masse, J. Zyss, A new approach in the design of polar crystals for quadratic nonlinear optics exemplified by the synthesis and crystal structure of 2amino-5-nitropyridinium dihydrogen monophosphate (2A5NPDP), J. Mol. Eng. 1 (1991) 141–152. [9] Z. Kotler, R. Hierle, D. Josse, J. Zyss, R. Masse, Quadratic nonlinear-optical properties of a new transparent and highly efficient organic–inorganic crystal: 2-amino-5-nitropyridiniumdihydrogen phosphate (2A5NPDP), J. Opt. Soc. Am. B 9 (1992) 534–547. [10] C.B. Aakeröy, K.R. Seddon, The hydrogen bond and crystal engineering, Chem. Soc. Rev. 22 (1993) 397–407. [11] T. Glowiak, S. Debrus, M. May, A.J. Barnes, H.K. Ratajczak, New molecular crystals with nonlinear optical properties: 3-aminophenol-H3 PO4 and 4-aminophenol-H3 PO4 , J. Mol. Struct. 596 (2001) 77–82. [12] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813.

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