J Mater Sci: Mater Electron DOI 10.1007/s10854-017-8122-9
Crystal growth and characterization of an efficient semi-organic nonlinear optical (NLO) donor-π-acceptor single crystal: 2-amino-5-nitropyridinium nitrate (2A5NPN) grown by slow evaporation solution technique (SEST) Sivasubramani Vediyappan1 · Mohankumar Vijayan1 · Senthil Pandian Muthu1 · Ramasamy Perumal1 · Martin Britto Dhas Sathyadhass Ambalapushpam2 Received: 24 August 2017 / Accepted: 23 October 2017 © Springer Science+Business Media, LLC 2017
Abstract The semi-organic nonlinear optical 2-amino5-nitropyridinium nitrate (2A5NPN) single crystals were successfully grown by slow evaporation solution technique with the dimension of 10 × 10 × 10 mm3. The crystalline structure of 2A5NPN was confirmed by single crystal X-ray diffraction measurement. The optical quality of the grown crystal was studied using UV–Vis–NIR spectrum analysis. It shows that the grown crystal has 69% of transmittance with the lower cut-off wavelength of 404 nm. The refractive index of 2A5NPN single crystal was calculated using the Prism-coupling method. The thermal diffusivity was analyzed using photoacoustic measurement. The dark and photoconductivity of the grown crystal were analyzed and the result shows that the crystal has negative photoconductivity nature. The mechanical strength of 2A5NPN crystal was analyzed by Vickers microhardness tester and the hardness number (Hv), Meyer’s index (n), yield strength (σy), stiffness constant (C11) and Hays–Kendall relation were evaluated. The fundamental solid state parameters such as valence electron plasma energy (ћωp), Penn gap ( Ep), Fermi energy ( Ef) and various type of electronic polarizability (α) of the grown crystal were determined using the dielectric parameters. The HOMO–LUMO energy and first-order hyperpolarizability (β) of 2A5NPN molecule were calculated in the gas phase by density functional theory. The laser damage threshold measurement has been performed and it reveals that the optical damage tolerance power of the grown crystal is high * Sivasubramani Vediyappan
[email protected] 1
SSN Research Centre, Sri Sivasubramaniya Nadar College of Engineering, Chennai, Tamilnadu 603 110, India
Department of Physics, Sacred Heart College, Vellore, Tirupattur, Tamilnadu 635 601, India
2
compared to other organic and inorganic NLO crystals. The third-order nonlinearity of 2A5NPN was assessed using an open and closed aperture Z-scan technique. The above results show that 2A5NPN crystal is a potential candidate for nonlinear optical device applications.
1 Introduction In the modern era of science, nonlinear optical (NLO) single crystals are highly demanded by scientists and crystal growth researchers due to their wide range of applications in the field of laser technology, optoelectronics, information processing, 3D optical data storage, optical limiters and optical switching applications [1–3]. There has been considerable effort to develop organic, inorganic and semi-organic NLO single crystals for certain types of device applications. The selection of crystals depends not only on NLO properties, but also on the physical properties such as optical, mechanical, thermal, electrical and laser damage threshold (LDT). In general, the organic molecules show large NLO response compared to that of inorganic materials, due to the presence of donor–acceptor groups with π-conjugated hydrogen bonding. Due to the poor optical, mechanical and thermal properties, the organic single crystals were limited for the device fabrications. In recent days, extensive efforts have been taken to develop new semi-organic NLO single crystals by combining organic and inorganic materials. It possesses several attractive physico-chemical properties such as high LDT, good mechanical property, wide optical transmission range and superior NLO activity, which make them suitable for optical device applications [4, 5]. The organic 2-amino-5-nitropyridine (2A5NP) stable molecule was invented and the series of derivatives were developed subsequently by Masse and Zyss [6, 7]. Among
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the organic and inorganic crystals, organic 2A5NP derivative single crystals have attracted considerable attention due to the presence of π-conjugated donor–acceptor groups that can induce high NLO behavior, where the nitro group is an electron acceptor and amino group is an electron donor. The pyridine ring acts as a cationic bonding site, the pyridinium nitrogen as a proton acceptor and the amino group as a proton acceptor. Because of this special molecular structure, it has been commonly used as NLO molecular building block [8]. In the recent past, 2A5NP derivatives have been widely investigated by several research groups using the same 2A5NP cation with organic or inorganic anions in order to get large NLO response in the new material. Over last few decades, numerous 2A5NP derivative compounds of centro or noncentrosymmetric frameworks such as 2-amino-5-nitropyridinim p-phenolsulfonate (2A5NPP) [8], 2-amino-5-nitropyridinium p-toluenesulfonate (2A5NPT) [9], 2-amino-5-nitropyridinium l -tartrate (2A5NLT) [10], 2-amino-5-nitropyridinium sulfate (2A5NPS) [11], 2-amino-5-nitropyridinim tetrafluoroborate (2A5NPFB) [12], 2-amino-5-nitropyridinium dihydrogen phosphate (2A5NPDP) [13] and 2-amino-5-nitropyridinium chloride (2A5NPCl) [14] have been engineered. Most of the 2A5NP derivative compounds were formed by short and multiple hydrogen bonding and it causes the enhanced optical, mechanical, chemical and thermal properties in the grown crystals. For example, 2A5NPP and 2A5NPT organic single crystals have shown second harmonic generation (SHG) signal nearly 400 and 90 times that of standard potassium dihydrogen phosphate (KDP) material respectively, at the fundamental wavelength of 1064 nm by Kurtz–Perry powder technique [8, 9]. Among the 2A5NP derivatives, the researchers have mainly focused on SHG properties of the grown crystals. However, the third-order NLO material also can be used in optical limiting, photonics and optoelectronics device applications. Hence, the present work has been taken up with the aim of growing high-quality semi-organic 2-amino-5-nitropyridinium nitrate (2A5NPN) single crystals by SEST. The crystal structure was already reported by Bagieu-Beucher et al. [15]. The crystal growth and its preliminary characterizations were reported earlier [16–18]. The optical, thermal, dielectric and photoluminescence properties were presented in our earlier report [19]. In the present work, third-order NLO 2A5NPN single crystal was synthesized and good quality single crystals have been grown by slow evaporation solution technique (SEST) at room temperature. The investigations on bulk growth of 2A5NPN and its structural, optical, photoacoustic (PA), photoconductive, mechanical, solid state parameters and LDT analysis were presented. Additionally, the density functional theory (DFT) method was used to determine the frontier molecular orbitals and first-order hyperpolarizability (β) of the title compound. The third-order nonlinearity of the
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J Mater Sci: Mater Electron
grown crystal was studied by standard single beam Z-scan technique. From the Z-scan method the nonlinear refractive index (n2), nonlinear absorption coefficient (β) third-order nonlinear susceptibility (χ(3)) and second order-hyperpolarizability (γ) were calculated and the results are discussed.
2 Experimental procedures 2.1 Material synthesis and crystal growth by conventional method The 2A5NP compound is a weak Bronsted base which gains a proton from the involving strong or medium acids in the aqueous solution at pH 2 [33]
log P = log K + n log d
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J Mater Sci: Mater Electron
Fig. 7 a The variations of hardness number ( Hv) with applied load P. b (i) As grown crystal surface and (ii), (iii), (iv), (v), (vi) are indentation for 10, 25, 50, 75 and 100 g of loads, respectively. c The Mayer’s plot. d The variation of yield strength (σy) with applied load (P). e The variation of stiffness constant (C11) with applied load (P). f Plot of log P versus d 2
Hardness number (Kg/mm2)
a 65 60 55
62.4 Kg/mm2
50 45 40 35
Fully damaged
30 0
b
20
40
80
Actual fit Linear fit
1.8 1.6 1.4 1.2 1.0 0.8
Work hardening coefficient (n) = 1.8
0.6
20 18
Log (d)
e 1.00E+012
20.80 MN/m2
16 14 12 10 0
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
f
20
80
Stiffness constant (C11)
60
Load P (g)
6.00E+011
4.00E+011
2.00E+011
50 40
40
60
Load (g)
80
100
Actual curve Linear fit
70
8.00E+011
A1 = 0.0406 g/µm
30 20 10
0.00E+000
0
20
40
60
Load (g)
13
100
d 22
2.0
Yeild strength (MN/m2)
c 2.2
Log (P)
60
Load P (g)
80
100
0
600
900
1200
1500
d2 (µm)
1800
2100
2400
J Mater Sci: Mater Electron Table 4 Values of hardness number, penetration depth, yield strength and elastic stiffness constant of 2A5NPN single crystal Load P (g)
Hv (kg/mm2)
Penetration depth (µm)
Yield strength (GN/m2)
Stiffness constant (C11) × 1011
10 25 50 75 100
31.98727 35.39424 62.05557 62.41472 41.89286
3.43 5.16 5.52 6.74 9.50
10.66242 11.79808 20.68519 20.80491 13.96429
8.56 1.73 8.85 9.21 5.66
𝜎y =
Hv [1 − (n − 2)] 2.9
(
12.5 (n − 2) 1 − (n − 2)
)n−2 (7)
In the present case n 0.08 > 0.6 0.7 1.5 2 > 1.5–2.2 2.86 3.5 4 5
[48] [48] [49] [48] [49] [48] [50] [51] Present work [51]
a
1.59
Closed aperture 1.36
Normalized Transmittance
stage. The output far-field intensity (amplitude of the phase shift) was measured as a function of sample position. The open and closed aperture Z-scan measurement of 2A5NPN single crystal was performed by He–Ne laser at 632.8 nm as the excitation source with the beam diameter 0.5 mm. The source was delivered onto the crystal which was placed at the focus of the converging lens with focal length (f) 30 mm. The optical path length was measured to be 85 cm. In closed aperture measurement, 2 mm of aperture radius ( ra) was used. The experimental details and obtained results are summarized in Table 9. The variable transmitting light was measured using digital power meter (Model- Field master GS-coherent). The NLO values are varying depending on the position of samples under the focused Gaussian beam. The recorded normalized transmittance spectrum of 2A5NPN by closed and open apertures are depicted in Fig. 10a, b, respectively. Normally, closed aperture measurement is insensitive for nonlinear absorption and most sensitive for nonlinear refractive index. The schematic diagram of closed aperture (CA) (Fig. 11) qualitatively elucidates the transmittance changes and its relation to the nonlinear refractive index of the sample. For closed aperture measurement, the output transmittance depends on the aperture radius (2 mm) and it is kept constant for the entire process. The output transmittance of the Z-scan curve increases or decreases depending on the changes in refractive index and absorption coefficient of the sample. Moreover, the sample causes an additional focus or defocus, depending on whether nonlinear refraction is positive or negative [53]. If the material possesses negative nonlinear refractive index ( n2 0), the output transmittance behaves as the same analogy that gives rise to an opposite valley-peak configuration (which is indicated by an orange line in the
1.14
0.91
0.68
0.45 -15
-10
-5
0
5
10
15
5
10
15
Z (mm)
b 1.54
Normalized transmittance
Table 8 Comparison of laserinduced damage threshold of 2A5NPN crystal and some other known NLO crystals
Open aperture
1.43 1.32 1.21 1.10 0.99 -15
-10
-5
0
Z (mm) Fig. 10 a Closed aperture spectrum of 2A5NPN single crystal. b Open aperture spectrum 2A5NPN single crystal
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Table 9 The experimental details and obtained nonlinear optical results from closed and open Z-scan measurement for 2A5NPN single crystals
J Mater Sci: Mater Electron Third-order NLO parameters
Obtained values
Laser beam wavelength (λ) Lens focal length (f) Optical path length Beam radius of the aperture (ωa) Aperture radius (ra) Sample thickness (L) Effective thickness (Leff) linear transmittance aperture (S) Linear absorption coefficient (α) Linear refractive index (no) Nonlinear refractive index ( n2)
632.8 nm 30 mm 850 mm 3.3 mm 2 mm 0.5 mm 0.465 mm 0.52 290.17 1.55 2.44214 × 10−11 m2/W 1.21 × 104 m/W 1.506 × 10− 09 esu 2.422 × 10−08 esu 2.427 × 10−08 esu 6.23 × 10−32 esu
Nonlinear absorption coefficient (β) Real part of the third-order susceptibility (Re (χ(3))) Imaginary part of the third-order susceptibility (Im (χ(3))) Third-order nonlinear optical susceptibility (χ(3)) Second-order molecular hyperpolarizability (γ)
) ( S = 1 − exp −2ra2 ∕𝜔2a
(26)
where ra is the radius of the aperture, 𝜔a is radius of transmitted laser beam at open aperture. The nonlinear refractive index is determined by the following relation.
n2 =
Fig. 11 Schematic diagram of closed aperture spectrum
ΔΦ0 KI0 Leff
, Leff is the effective thickness of the sample where K = 2𝜋 𝜆 and I0 is the intensity of the laser beam at the focal point (I0 = 26.50 MW/m2). The effective thickness of 2A5NPN single crystal is calculated from the following relation.
Leff = Fig. 11), which indicates the self-focusing effect and this is attributed to the thermal nonlinearity resulting from linear absorption [54]. The obtained closed aperture curve clearly shows that the pre-focal minimum followed by post-focal maximum, which is a signature of positive nonlinear refractive index and self-focusing lensing effect of the grown crystal. It is an essential property for the application in the protection of optical sensors [55]. The nonlinear refractive index (n2) was calculated from the transmission of difference between peak and valley value in the closed aperture by the following relation.
ΔT p − v = 0.406(1 − S)0.25 ||ΔΦ0 || (25) Here ||ΔΦ0 || is the one axis phase shift, S is the linear aperture transmittance (S = 0.52) and it can be obtained from the following relation.
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(27)
[1 − exp(−𝛼L)] 𝛼
(28)
where α is the linear absorption coefficient. It can be calculated from UV–Vis–NIR transmittance data and L is thickness of the crystal. From the open aperture Z-scan curve, the third-order nonlinear absorption coefficient (β) can be calculated using the following relation.
√ 2 2 ΔT 𝛽= I0 Leff
(29)
where ∆T is the valley value in the open aperture at sample position (Z = 0). The value of sign β will be negative for saturated absorption and a positive sign for two-photon absorption processes [56]. The real and imaginary parts of third-order NLO susceptibility (𝜒 (3)) were calculated and it was determined from the experimental values such as
J Mater Sci: Mater Electron
third-order nonlinear refractive index (n2) and third-order nonlinear absorption coefficient (𝛽 ).
Re 𝜒
(3)
) ( 10-4 (𝜀0 C2 n20 n2 ) cm2 (esu) = 𝜋 W
Img 𝜒 (3) (esu) =
10-2 (𝜀0 C2 n20 𝜆𝛽) ( cm ) 4𝜋 2
W
(30)
(31)
Here 𝜀0 is the absolute value of permittivity in vacuum (8.851 × 10−12 F/m) and C (3 × 108 m/s) is the velocity of light in vacuum. n0 is the linear refractive index of the grown crystal and it has been calculated using Metricon Model 2010/M Prism coupler instrument with the laser light wavelength of 632 nm. The value was found to be 1.55. The absolute third-order NLO susceptibility is easily obtained by the following relation.
[ ] 2 2 1∕2 | 3| esu |𝜒 | = (Re(𝜒 (3) )) + (Im(𝜒 (3) )) | |
(32)
The open aperture is insensitive for nonlinear refraction analysis and the nonlinear absorption is negligible. In the open aperture configuration, the magnitude of intensity depends on nonlinear absorption and it exhibits a high intensity of transmittance with respect to the focus (z = 0). Presence of saturable absorption with positive absorption coefficient in the material indicates that it can be useful for third-order NLO applications [57, 58]. Further, the second-order hyperpolarizability (γ) of 2A5NPN molecules was estimated through the following equation.
Re [𝛾] =
Re (𝜒 (3) ) Nf 4
(33)
where N is the number of molecules per unit volume and it can be obtained from following equation.
N=
𝜌 ∗ (NA ) M
(34)
where 𝜌 ∗ is the density of the crystal, NA is the Avogadro number, M is the molecular weight. The local field correction factor (f) can be obtained from the following equation.
f =
(n20 + 2) 3
(35)
Here, no is the linear refractive index of the grown crystal. Additionally, the coupling factor (P) was calculated in the ratio of the imaginary and real part of third-order susceptibility of 2A5NPN crystal.
P=
Im (𝜒 (3) ) Re (𝜒 (3) )
(36)
The coupling factor was found to be 16.07. It is due to the contribution of nonlinear absorption change which is more dominant than the nonlinear refraction and the electronic origin of nonlinearity [59]. In order to know the suitability of the grown crystal for optical switching applications, two figures of merit were evaluated using the following equations.
W=
n2 I0 𝛼𝜆
(37)
T=
𝛽𝜆 n2
(38)
where n2 is nonlinear refractive index, I0 is the intensity of the laser beam (I0 = 26.50 MW/m2), α is linear absorption coefficient, λ is wavelength of laser source and β is nonlinear absorption coefficient. For optical switching applications, the value of figure of merit W >> 1 and T
