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PS polymer nanocomposite synthesized by chemical route
Optics Communications 352 (2015) 55–62
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Investigation of non-linear optical properties of CdS/PS polymer nanocomposite synthesized by chemical route S.K. Tripathi n, Ramneek Kaur, Jyoti Department of Physics, Center of Advanced Study in Physics, Panjab University, Chandigarh 160 014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2014 Received in revised form 3 April 2015 Accepted 17 April 2015 Available online 27 April 2015
Cadmium Sulfide (CdS) nanoparticles play an important role in non-linear optoelectronic devices. CdS/ Polystyrene(PS) nanocomposite has been prepared by chemical ex-situ route and characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet–visible (UV–vis) and Photoluminescence (PL) spectroscopy. XRD spectra of CdS/PS nanocomposite reveals the cubic phase of CdS nanoparticles with average crystallite size 2.54 nm. The vibrational band corresponding to Cd–S bond has been observed at 406.57 cm 1 in FTIR spectra of CdS/PS nanocomposite along the typical styrene bonds. Quantum confinement effect in the CdS/PS nanocomposite has been confirmed from the UV–vis spectra. In PL emission spectra, in addition to band to band transition emission, the green and yellow bands have been observed due to the interstitial sulfur and cadmium defect states respectively. Z-scan technique has been utilized to study the non-linear optical properties of the CdS/PS nanocomposite. The value of non-linear absorption coefficient (β) and non-linear refractive index (n2) has been calculated. The large value of third order non-linear susceptibility is due to the quantum confinement effect plus the thermal lensing effect produced across the sample. & 2015 Published by Elsevier B.V.
Keywords: CdS Non-linearity Polymer nanocomposite Z-scan
1. Introduction Recently, polymer nanocomposites (PNCs) of II–VI semiconductor compounds have been receiving great interest due to their controllable chemical, physical and electronic properties [1– 3]. In PNCs, polymer not only stabilizes the nanoparticles (NPs) but also modifies the surface of the NPs by passivating the defect states and the dangling bonds. Also the polymer chains act as linkers and helps in achieving the extended framework of NPs in polymer matrix [4,5]. Among II–VI semiconductors, Cadmium sulfide (CdS) is a direct band gap material with bulk band gap value 2.42 eV and found applications in various fields such as photoconductive cells, optical waveguides, transducers and non-linear optical devices etc [6–9]. CdS NPs dispersed in polymer matrices have been widely studied. Among polymers, polystyrene (PS) has been widely used due to its low specific weight, mechanical flexibility, high chemical resistance and biocompatibility [10]. Various approaches had been reported for the preparation of nanoparticle/polymer composites. Wu et al. [11] had provided a novel microemulsion method for the synthesis of CdS/PS hollow spheres at the interface of water and oil by simultaneous γ-irradiation. The two main synthesis approaches n
Corresponding author. Fax: þ 91 172 2783336. E-mail addresses: [email protected], [email protected] (S.K. Tripathi).
http://dx.doi.org/10.1016/j.optcom.2015.04.042 0030-4018/& 2015 Published by Elsevier B.V.
used are; (1) in-situ technique: the NPs are generated in the presence of polymer and (2) ex-situ technique: dispersion of separately prepared NPs to the polymer matrix. Ex-situ technique offers the advantage over in-situ technique as it enables the control over the amount of nanoparticle loading in the polymer matrix. Tamborra et al. [12] had reported the synthesis of CdS nanocrystals embedded in PS matrix by ex-situ technique. In present work, exsitu technique has been used for the synthesis of nanocomposite (NC). The demand of non-linear optical materials is increasing due to their tremendous applications in optoelectronic devices. The study of third order susceptibility is of great interest due to its importance in various applications such as optical switching, optical fibers and optical limiting in II–VI semiconductors [13]. Bulk CdS shows strong optical non-linear behavior in the band gap region [14]. CdS also exhibits large second order non-linear coefficient [15]. Ren et al. [16] had studied the second order non-linearity of CdS nanowires incorporated with silver nanocavities and utilized it in optical router for nanophotonic systems. Liu et al. [17] had studied the effect of thermal/electrical poling on second-harmonic generation (SHG) in CdS-doped glasses. The two conditions that should be obeyed by the material to exhibit second order nonlinearity are: lack of center of symmetry, phase matching condition. In case of nanoparticles, the permanent dipole moment is not stable due to their spherical shape with size less than the lower limit requisite in SHG [18,19]. These conditions limit the second
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order non-linear optical study of the materials. In present work, the third order non-linear optical parameters of CdS nanocomposite have been studied. Li et al. [20] had studied the bound-electronic and the free carrier optical non-linearity by single beam Z scan technique operating at 532 nm in 50 μm thick hexagonal CdS single crystal using 4.8 GW/cm2 input irradiance. Todd et al. [21] had found the value of non-linear absorption coefficient (β) and non-linear refractive index (n2) to be 3.2 cm/GW and 50 10 12 esu respectively for CdS using laser light of 610 nm. Chin et al. [22] had studied the non-linear optical properties of hybrid nanocomposites of CdS NPs using Nd:YAG laser of wavelength 532 nm. They observed the value of non-linear refractive index increases linearly from 1.0 10 4 to 3.0 10 4 cm2/GW as the input irradiance decreases. The non-linear optical measurements for liquid samples of CdS NPs in heptane solution had also been studied by Souza et al. [23] using 1 mm thick quartz cuvette. He et al. [24] had reported the large value of third order non-linear susceptibility ( 3.4 10 11 esu) for CdS NCs in polymeric film. In the present work, the simple processing technique has been used for the synthesis of CdS/PS nanocomposite. PVP has been used as a steric stabilizer during synthesis. The structural and optical properties of the nanocomposite have been studied with X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), UV–vis absorption and Photoluminescence (PL) spectroscopy. The non-linear optical properties of the CdS/PS nanocomposite have been studied using Z-scan technique.
2. Experimental details 2.1. Chemicals used Cadmium acetate (Cd(CH3COO)2) and benzene were purchased from Qualigens fine chemicals, Sodium sulfide (Anhydrous purified) and Ammonia were purchased from Merck Specialities Pvt. Ltd., Polyvinyl pyrrolidone (PVP) and Polystyrene (PS) were obtained from Sigma Aldrich. Deionized water was used in all experiments and all of the other chemicals were of analytical grade and used as purchased without any further purification. 2.2. Synthesis of CdS/PS nanocomposite CdS/PS nanocomposite has been prepared by chemical route using ex-situ technique. It involves two steps: (1) Preparation of CdS NPs (2) Dispersion of CdS NPs into PS matrix. Cd(CH3COO)2 and sodium sulfide were used as a cadmium source and sulfur source, respectively. Cadmium source solution was prepared in 30 ml deionized water by dissolving 2.8 g of Cd(CH3COO)2 to it. Sulfur source was prepared separately in 30 ml deionized water by dissolving 820 mg of sodium sulfide to it. CdS NPs were generated by dropwise addition of sodium sulfide solution to cadmium acetate solution under continuous stirring. The pH of the solution was adjusted to 11 by dropwise addition of aqueous ammonia. After 5 min of stirring, 0.5 g of PVP was added and solution was left for 2 h for stirring. The prepared CdS NPs were collected, dried and crushed to obtain the fine powder. Yellow colored CdS NPs were obtained after crushing. Polystyrene solution was prepared by adding 3 g of polystyrene to 100 ml of benezene. For the preparation of CdS/PS nanocomposite, 500 mg of CdS NPs were redissolved in 10 ml of benzene. CdS nanoparticle solution was added dropwise to the 50 ml polystyrene solution with continuous stirring at room temperature to obtain CdS/PS nanocomposite. For XRD analysis, the thin films of the CdS/PS nanocomposite were deposited on glass substrate by solution casting method at room temperature.
2.3. Characterization XRD patterns were recorded by using Phillips PW-1710 X-ray diffractometer with CuKα radiation in the 2θ range from 5° to 90°. FTIR spectra of the samples were recorded using PerkinElmer PERX 1 FTIR spectrophotometer with spectral resolution 1 cm 1. UV absorption spectra were recorded using single beam Spectroscan 30 spectrometer in the wavelength range of 300–800 nm. Photoluminescence properties of the samples were recorded on a Shimadzu Spectrofluorophotometer RF-5301PC. The non-linear optical properties of the CdS/PS nanocomposite were studied by using Z-scan technique. A continuous wave (CW) He–Ne laser operating at a wavelength of 632.8 nm was used to perform open and closed aperture Z-scan experiment. During Z scan measurements, the sample was translated along the z-direction (in the direction of wave propagation) around the focal point by keeping the wavelength of laser light fixed. The non-linear absorption coefficient (β) and the imaginary part of the third order non-linear (3) ) were obtained from the open aperture Z-scan susceptibility ( χIm measurements. In case of closed aperture Z-scan scheme, the magnitude of peak-valley transmission gives the measure of nonlinear refractive index (n2) and the real part of third order nonlinear susceptibility ( χR(3) ). To perform intensity dependent Z-scan measurements, a set of polarizer and analyzer was used in front of laser source. All the Z-scan measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD pattern of PS and CdS/PS NC thin films, where the scattered intensity has been plotted as a function of diffraction angle (2θ) in the range of 5–90°. XRD pattern of PS shows a broad peak at 2θ ¼19.27° with a small hump at 2θ ¼10.29° indicating the amorphous nature of polymer [25]. With the addition of CdS NPs into the PS matrix, the crystalline nature of the nanocomposite has been obtained. XRD pattern of CdS/PS NC exhibit prominent broad peaks centered at 2θ ¼26.46°, 43.59° and 51.04°. These peaks correspond to the cubic phase of CdS with diffraction planes [111], [220] and [311] respectively [26], [JCPDS file no. 00-10-454]. The weak intensity peak at 2θ ¼33.25° indicates the [200] plane of cubic CdS [JCPDS file no. 00-10-454]. The lattice parameter ‘a’ has been determined by using relation [27];
1 (h2 + k 2 + l2) = 2 d a2
Fig. 1. XRD spectra of PS and CdS/PS nanocomposite.
(1)
S.K. Tripathi et al. / Optics Communications 352 (2015) 55–62
where h, k and l represent the lattice planes. The value of ‘a’ calculated from the ‘d’’ value corresponding to [111] plane is 5.827 Å. The crystallite size has been calculated by using Debye Scherrer's formula [28];
D=
kλ β cos (θ )
(2)
where β is the full width at half maxima (FWHM), θ is the diffraction angle, λ is the X-ray wavelength and k is Scherrer's constant of the order of unity. The diffraction peaks are considerably broad indicating a very fine size of the NPs. The diffraction peak broadening may be due to the inhomogeneous strain present in the films [29].The value of strain (ɛ) has been calculated by using following relation [30];
ε=
β 4 tan (θ )
(3)
Dislocation density (δ) which is a measure of total length of dislocation per unit volume of the crystal and has been determined by using following formula [31];
δ=
n D2
(4)
here factor n is equal to unity, giving the minimum density of dislocation. The values of 2θ, crystallite size, lattice constant, strain and dislocation density corresponding to each plane are given in Table 1. The average crystallite size and average strain calculated by using Eqs. (2) and (3) is 2.54 nm and 7.32 10 1 respectively. This is due to the structural disorderness which occurs during the thin film formation, when the adjacent grains come into contact. Also, the thermal expansion coefficient differences between the substrate and thin film may contribute to the strain [32]. 3.2. Fourier transform infrared spectroscopy FTIR spectra of the PS and CdS/PS nanocomposite are shown in Fig. 2. The various functional groups present in the PS and CdS/PS nanocomposite corresponding to their wavenumber are shown in Table 2. The presence of typical styrene bands at 665.29 cm 1, 701.20 cm 1, 759.63 cm 1, 1034.76 cm 1, 1175.94 cm 1 and 3034.95 cm 1 is due to the polystyrene in the nanocomposite [33]. 1175.94 cm 1, The absorption bands at 701.20 cm 1, 1 1 and 3070.81 cm can be assigned to the asym1526.35 cm metric and symmetric stretching vibrations of ¼C–H, C–O, C ¼ C and C–H groups respectively. The characteristic vibrational absorption peak for the Cd–S bond has been observed at 406.57 cm 1 in CdS/PS nanocomposite, but the intensity of this peak is weak [34]. 3.3. Absorption spectroscopy Fig. 3(a) shows the absorption spectra of PS and CdS/PS nanocomposite. PS shows the absorbance only in the UV region but with the addition of CdS NPs the absorbance shifted to visible region. CdS/PS nanocomposite shows an absorption peak at 441 nm which is due to the optical transition of Ist excitonic state in the CdS NPs [35]. The corresponding band gap of the CdS/PS Table 1 Crystallite size, strain and dislocation density calculated from XRD data. 2 θ (deg)
hkl
D (nm)
ε
δ ( 1018)
a (Å)
26.46 43.59 51.04
111 220 311
5.28 1.63 0.72
0.03 0.06 0.12
0.04 0.38 1.93
5.83 5.86 5.93
57
nanocomposite has been calculated by using relation Eg (eV) ¼ (1238/λ (nm)). The band gap of the CdS/PS nanocomposite calculated from above relation is 2.81 eV, whereas the bulk band gap of CdS is 2.42 eV. The increase in the band gap value of CdS/PS nanocomposite confirms the Quantum confinement effect. As we know that the Quantum confinement is responsible for the increase of energy difference between the energy states and band gap. PS does not show any absorption peak, therefore its band gap value has been calculated by using Tauc's relation which relates the absorption coefficient (α) to band gap as;
(αhν )1/ n = A (hν − Eg )
(5)
where Eg is the optical band gap energy, ν is the frequency, h is Planck's constant, A is a constant, and n is the constant parameter which is equal to 1/2 for direct allowed transition, 2 for indirect allowed transition, 3/2 for direct forbidden transition and 3 for indirect forbidden transition. Using Tauc's relation the band gap value calculated for PS is 3.93 eV as shown in Fig. 3(b). The size of the NPs has been calculated from the band gap information by using effective mass approximation (EMA) method [36] given by following formula;
⎛ ℏ2π 2 ⎞ ⎛ 1 1 ⎞ 1.8e2 ⎟⎜ ⎟− Egn = Egb + ⎜ + 2 * εR mh* ⎠ ⎝ 2R ⎠ ⎝ m e
(6)
where Egb is the bulk band gap value, Egn is the nanoparticle band gap value taken from absorption data, me* is the effective electron mass, mh* is the effective hole mass and ε is the dielectric constant of the medium. The second term in Eq. (6) represents the quantum localization energy whereas the third term corresponds to the Coulomb potential energy, respectively [36]. The coulomb term for electron–hole interaction is very small as compared to the Quantum confinement term, thus usually neglected. For CdS, me* = 0.21me and mh* = 0.80me where me is the free electron mass [36]. The value of particle radius (R) calculated from Eq. (6) is 2.41 nm (particle diameter, 2R ¼4.82 nm), which is smaller than bulk exciton radius (3 nm) [37] which indicates the strong confinement in the CdS NPs. 3.4. Photoluminescence properties PL spectra of the samples have been recorded at an excitation wavelength of 320 nm and shown in Fig. 4. PL spectra of PS shows the emission peak around 363 nm. This is due to the generation of luminescent substances such as benzoic acid, styrene, methybenzonate, double bond in the main chain, stilbene, etc. These are produced by reaction of oxygen with radicals, formed from the elimination of hydrogen atom from the carbon adjacent to the phenyl group of PS under light illumination [38]. In the PL spectra of CdS/PS nanocomposite, several emission bands has been observed at 430 nm (2.87 eV), 473 nm (2.62 eV) and 563 nm (2.19 eV). Generally in semiconductor nanomaterials, the various mechanisms contributing to the PL emission are: (a) band edge emission, (b) deep trap emission, (c) surface defect emission and (d) the nanoparticle size distribution [39]. The emission band at 2.87 eV is in the vicinity of calculated band gap from absorbance spectra and is due to the band to band transition. In CdS NPs, the four types of point defects: interstitial cadmium (ICd), interstitial sulfur (IS), cadmium vacancy (VCd) and sulfur vacancy (VS) exists which play an important role in photoluminescence spectra [40]. The green and yellow emission bands at 473 nm and 563 nm are due to the interstitial sulfur and interstitial cadmium defect states respectively [41]. Fig. 5 shows the schematic representation of different transitions involved in PL spectra. In case of CdS/PS nanocomposite, the CdS NPs plus the luminescent substances in PS matrix are contributing to the PL emission.
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Fig. 2. FTIR spectra for PS and CdS/PS nanocomposite.
Fig. 4. Photoluminescence spectra of PS and CdS/PS nanocomposite excited at an excitation wavelength of 320 nm.
Table 2 Assignment of functional groups for FTIR spectra. Wave number (cm 1)
Functional group
CdS/PS NC
PS
406.57 665.29 701.20 759.63 846.44 1034.76 1175.94 1393.69 1477.77 1526.35 1814.82 1959.82 3034.95
– 665.39 700.83 759.13 846.28 1034.83 1176.63 1393.11 1477.79 1526.14 1814.74 1960.05 3034.96
Cd–S Phenyl group ¼ C–H ¼ C–H ¼ C–H C–O C–O –C–H C ¼C C ¼C C ¼O C ¼C C–H
3.5. Non-linear optical properties of CdS/PS nanocomposite The third order non-linear optical properties of the CdS/PS nanocomposite have been investigated by Z scan technique. The simplicity of the Z scan technique is that the non-linear parameters such as n2, β and third order non-linear susceptibility (χ(3))
Fig. 5. Schematic representation of various transitions involved in photoluminescence spectra.
can be obtained from the single experiment. Fig. 6(a) and (b) shows the open aperture Z scan curve and the variation of q0(zs) with sample position, where q0(zs) is the parameter determining the strength of the non-linearity. The value of q0(zs) has been determined from the normalized transmittance T (zs ) (OA Z scan) by using following relations which are valid for T (zs ) Z0.224 and T (zs ) Z0.243 for Gaussian and hyperbolic secant pulses respectively:
Fig. 3. (a) UV–vis spectra of PS and CdS/PS nanocomposite and (b) variation of (αhv)2 vs. hv for PS.
S.K. Tripathi et al. / Optics Communications 352 (2015) 55–62
59
Fig. 6. (a) Open aperture Z-scan curve for CdS/PS nanocomposite and (b) variation of q0(zs) with sample position.
⎧ 2 3 ⎪a 0 + a1T (zs ) + a 2 T (zs ) + a 3 T (zs ) for T (zs ) ≤ 0.75 q0 (zs ) = ⎨ ⎪ c 2 c0 + c1 [T (zs )] for T (zs ) ≥ 0.75 ⎩
(7)
here the coefficients a0, a1, a2, a3, c0, c1, c2 have values 15.66, 37.45, 30.76, 8.97, 2.301, 2.156, 1.563 for Gaussian pulses. As seen from the open aperture Z-scan (Fig. 6(a)), the CdS/PS nanocomposite exhibits reverse saturable absorption. With increasing laser beam intensity, the transmittance of the nanocomposite increases. The value of β has been calculated from open aperture Z scan curve by using following relation [42];
λzR Q 0 β= 2P0 Leff
(8)
Leff = (1 − exp ( − αL ))/α
(9)
here L is the sample length and α is the linear absorption coefficient. The value of Q0 and zR has been calculated from Fig. 6(b). The maximum value of q0(zs) at z ¼z0 gives the value of Q0 whereas from the FWHM of this graph, the value of zR has been calculated, as at zs = z0 ± zR , the value of q0(zs) is equal to Q0/2. Fig. 7 shows the closed aperture Z scan curves for CdS/PS nanocomposite with a signature of peak followed by a valley indicating the negative type of non-linearity in the nanocomposite. From closed aperture measurements, the value of n2 has been calculated by using following relation [43];
Δφ0 λ
2πI0 Leff
(10)
where |Δφ0 | represents the phase change of the laser beam due to non-linear refraction and Io is the input irradiance of laser light at focal point. The value of |Δφ0 | has been calculated from the difference between the normalized peak and valley transmittances (ΔTp v) by using following relation [44];
Δφo =
χ (3) =
(3) 2 (χR(3) )2 + (χIm )
where,
χR(3)
is the real part of
(12) χ (3)
and
(3) is χIm
the imaginary part of
χ (3) and given as;
where λ is the laser wavelength (632.8 nm), P0 is the peak power of the pulses and Leff is the effective thickness of the sample, given by the relation
n2 =
The non-linear behavior of the CdS/PS nanocomposite is due to the CdS NPs as the pure PS does not show any non-linear behavior [22]. The values of β and n2 calculated for CdS/PS nanocomposite using Eqs. (8) and (10) are given in Table 3 and are in good agreement with the literature results [45]. Furthermore, the value of χ(3) has been determined from β and n2 values by using following relations;
ΔTp − v
0.406 (1 − S )0.25
Here S ( 0.39) is the linear transmittance of the aperture.
(11)
χR(3) = 2n2 n2ε 0 c
(13)
(3) χIm = ε 0 n2cλβ/(3π )
(14)
here n is the linear refractive index of the sample (2.213), ε0 is the permittivity of free space (8.854 10 14 F/cm) and c is the velocity (3) calculated using Eqs. (13) and (14) of light. The value of χR(3) and χIm 13 18 is 1.6 10 and 3.5 10 m2/V2, respectively. Table 4 shows (3) ) for CdS/PS the comparison of non-linear parameters (β, n2, χIm nanocomposite with the literature results. It has been found that the value of n2 is comparable to the value reported by Rao et al. [45] for TGA (thioglycerol) capped CdS NPs with size 4.5 nm. But this value is much smaller than the value reported by Chin et al. [22] for CdS/PS nanocomposite by approximately 5 orders. The large difference in optical non-linearity in CdS/PS nanocomposite is due to the Quantum confinement effect as observed from the absorption spectra (Fig. 3(a)). In nanosize regime, due to the high surface area activity and the special surface structure leads to the change in optical properties with the surrounding environment [42,50]. Under laser light of wavelength 632.8 nm, the enhancement of local electric field inside the NPs contribute to the optical non-linear behavior of the nanocomposite. Mamta et al. [42] had reported that the surface states present in the nanocomposite and the dielectric confinement effect also contributes to the large optical non-linearity. In a particular experiment, the various physical mechanisms contributing to the non-linear refractive index are electronic, vibrational, electrostriction and thermal, depending on the wide range of time scales as;
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Fig. 8. Variation of n2 with laser beam intensity.
Fig. 7. Closed aperture Z-scan curve for CdS/PS nanocomposite. Table 3 Variation of non-linear parameters and thermo optical coefficient with laser beam intensity. I0 ( 102 W/cm2)
β ( 10 8 cm/W)
n2 ( 10 8 cm2/W)
(3) ( 10 18 m2/V2) χIm
dn/dT ( 10 8 K 1)
0.82 1.02 1.59 3.98
40.1 34.1 21.2 4.8
14.5 11.9 8.6 3.9
3.5 2.9 1.8 0.42
1.81 1.48 1.06 0.48
n2 = n2 (electronic) + n2 (vibrational) + n2 (electrostriction) + n2 (thermal)
(15)
The time response for the electronic, vibrational and electrostrictive contribution is 10 15, 10 13 and 10 8 s respectively [51]. The contribution of thermal effects is at much longer time scales (tens of μs). In case of nanosecond laser pulses, only the first two mechanisms contribute to the non-linear properties. In the present work, CW He–Ne laser is used, thus the main contribution to the non-linearity of the nanocomposite is due to the thermal effects [52]. Due to the temperature gradient effect produced by thermal heating, the refractive index change occurs across the sample resulting in the lensing effect in the sample [53]. The thermal effect changes depend on the laser beam intensity. To confirm the presence of thermal effects, the intensity dependent Z-scan measurements have been performed (Figs. 6 and 7). The thermo optical coefficient (dn/dT) has been calculated by using following relation [23]:
dn λκ =− Δφo dT Pαo Leff
(16)
where κ is the thermal conductivity (0.09 W cm 1 K 1), Δφo is the
on-axis phase shift, P is laser power. With increasing laser beam intensity, the transmittance and the difference between Tp and Tv increases resulting in increase in n2 value as shown in Fig. 8. With increasing laser beam intensity, the non-linear refractive index and thermo optical coefficient increases indicating the presence of thermo optical effects. The values calculated are given in Table 3. Also the separation between the peak-valley increases with increasing laser beam intensity indicating the non Kerr type non-linearity and the presence of the thermal component in the non-linear behavior of CdS/PS nanocomposite [53]. Thus, the combination of Quantum confinement effect and the thermal effect results in large non-linear optical response of CdS/PS nanocomposite. The enhanced UV–vis absorption and PL intensity makes CdS/PS nanocomposite suitable for electroluminescent devices. The large value of χ 3is responsible for its use in various non-linear optical devices.
4. Conclusion CdS/PS nanocomposite has been synthesized by ex-situ technique using benzene solvent. The cubic phase of CdS NPs has been confirmed from the XRD spectra with average crystallite size
Table 4 (3) Comparison of non-linear optical parameters (β, n2 and χIm ) with the literature.
Sample (Size)
Size (nm)
Wavelength (nm)
β (cm/W)
n2 (cm2/W)
(3) (m2/V2) χIm
Reference
CdS/PS NC (I0 ¼3.98 102 W/cm2) Bulk CdS CdS NPs CdS NPs CdS/PS NC CdS NPs CdS NPs CdS/Sodium borosilicate
4.8 – 4.5 5–7 3–6 3.3 3.0 10–20
632.8 532 532 532 532 530 532 770
4.8 10 8 5.4 10 9 3.1 10 8 7.6 10 10 – – 6.0 10 11 6.3 10 9
3.9 10 8 5.3 10 13 6.9 10 8 – 1.0 10 13 3.0 10 14 – 2.2 10 12
4.2 10 19 – – 2.9 10 22 – – 2.6 10 22 1.7 10 18
Present work [20] [45] [46] [22] [47] [48] [49]
S.K. Tripathi et al. / Optics Communications 352 (2015) 55–62
2.54 nm. In FTIR spectra, the characteristic vibrational band for Cd–S bond at 406.57 cm 1 has been observed along the typical styrene bands. The absorption region of PS has been found to extend from UV region to visible region with the addition of CdS NPs to it. Photoluminescence spectra of CdS/PS nanocomposite consists of band to band transition emission plus the green and yellow emission bands due to the defect states present in it. CdS/ PS nanocomposite shows good non-linear optical properties with β and n2 values 4.8 10 8 cm/W and 3.9 10 8 cm2/W respectively. CdS/PS nanocomposite synthesized by simple processing technique with good optoelectronic properties may be useful for the fabrication of novel optical sensors and electronic devices.
Acknowlegements This work is financially supported by University Grant Commission (UGC) (Major Research Project) [F. No. 42-781/2013(SR)], N. Delhi. Ms. Ramneek Kaur is thankful to UGC, N. Delhi for providing the fellowship.
[20] [21] [22]
[23]
[24]
[25]
[26]
[27]
[28]
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Investigation of non-linear optical properties of CdS/PS polymer nanocomposite synthesized by chemical route S.K. Tripathi n, Ramneek Kaur, Jyoti Department of Physics, Center of Advanced Study in Physics, Panjab University, Chandigarh 160 014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 19 November 2014 Received in revised form 3 April 2015 Accepted 17 April 2015 Available online 27 April 2015
Cadmium Sulfide (CdS) nanoparticles play an important role in non-linear optoelectronic devices. CdS/ Polystyrene(PS) nanocomposite has been prepared by chemical ex-situ route and characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet–visible (UV–vis) and Photoluminescence (PL) spectroscopy. XRD spectra of CdS/PS nanocomposite reveals the cubic phase of CdS nanoparticles with average crystallite size 2.54 nm. The vibrational band corresponding to Cd–S bond has been observed at 406.57 cm 1 in FTIR spectra of CdS/PS nanocomposite along the typical styrene bonds. Quantum confinement effect in the CdS/PS nanocomposite has been confirmed from the UV–vis spectra. In PL emission spectra, in addition to band to band transition emission, the green and yellow bands have been observed due to the interstitial sulfur and cadmium defect states respectively. Z-scan technique has been utilized to study the non-linear optical properties of the CdS/PS nanocomposite. The value of non-linear absorption coefficient (β) and non-linear refractive index (n2) has been calculated. The large value of third order non-linear susceptibility is due to the quantum confinement effect plus the thermal lensing effect produced across the sample. & 2015 Published by Elsevier B.V.
Keywords: CdS Non-linearity Polymer nanocomposite Z-scan
1. Introduction Recently, polymer nanocomposites (PNCs) of II–VI semiconductor compounds have been receiving great interest due to their controllable chemical, physical and electronic properties [1– 3]. In PNCs, polymer not only stabilizes the nanoparticles (NPs) but also modifies the surface of the NPs by passivating the defect states and the dangling bonds. Also the polymer chains act as linkers and helps in achieving the extended framework of NPs in polymer matrix [4,5]. Among II–VI semiconductors, Cadmium sulfide (CdS) is a direct band gap material with bulk band gap value 2.42 eV and found applications in various fields such as photoconductive cells, optical waveguides, transducers and non-linear optical devices etc [6–9]. CdS NPs dispersed in polymer matrices have been widely studied. Among polymers, polystyrene (PS) has been widely used due to its low specific weight, mechanical flexibility, high chemical resistance and biocompatibility [10]. Various approaches had been reported for the preparation of nanoparticle/polymer composites. Wu et al. [11] had provided a novel microemulsion method for the synthesis of CdS/PS hollow spheres at the interface of water and oil by simultaneous γ-irradiation. The two main synthesis approaches n
Corresponding author. Fax: þ 91 172 2783336. E-mail addresses: [email protected], [email protected] (S.K. Tripathi).
http://dx.doi.org/10.1016/j.optcom.2015.04.042 0030-4018/& 2015 Published by Elsevier B.V.
used are; (1) in-situ technique: the NPs are generated in the presence of polymer and (2) ex-situ technique: dispersion of separately prepared NPs to the polymer matrix. Ex-situ technique offers the advantage over in-situ technique as it enables the control over the amount of nanoparticle loading in the polymer matrix. Tamborra et al. [12] had reported the synthesis of CdS nanocrystals embedded in PS matrix by ex-situ technique. In present work, exsitu technique has been used for the synthesis of nanocomposite (NC). The demand of non-linear optical materials is increasing due to their tremendous applications in optoelectronic devices. The study of third order susceptibility is of great interest due to its importance in various applications such as optical switching, optical fibers and optical limiting in II–VI semiconductors [13]. Bulk CdS shows strong optical non-linear behavior in the band gap region [14]. CdS also exhibits large second order non-linear coefficient [15]. Ren et al. [16] had studied the second order non-linearity of CdS nanowires incorporated with silver nanocavities and utilized it in optical router for nanophotonic systems. Liu et al. [17] had studied the effect of thermal/electrical poling on second-harmonic generation (SHG) in CdS-doped glasses. The two conditions that should be obeyed by the material to exhibit second order nonlinearity are: lack of center of symmetry, phase matching condition. In case of nanoparticles, the permanent dipole moment is not stable due to their spherical shape with size less than the lower limit requisite in SHG [18,19]. These conditions limit the second
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order non-linear optical study of the materials. In present work, the third order non-linear optical parameters of CdS nanocomposite have been studied. Li et al. [20] had studied the bound-electronic and the free carrier optical non-linearity by single beam Z scan technique operating at 532 nm in 50 μm thick hexagonal CdS single crystal using 4.8 GW/cm2 input irradiance. Todd et al. [21] had found the value of non-linear absorption coefficient (β) and non-linear refractive index (n2) to be 3.2 cm/GW and 50 10 12 esu respectively for CdS using laser light of 610 nm. Chin et al. [22] had studied the non-linear optical properties of hybrid nanocomposites of CdS NPs using Nd:YAG laser of wavelength 532 nm. They observed the value of non-linear refractive index increases linearly from 1.0 10 4 to 3.0 10 4 cm2/GW as the input irradiance decreases. The non-linear optical measurements for liquid samples of CdS NPs in heptane solution had also been studied by Souza et al. [23] using 1 mm thick quartz cuvette. He et al. [24] had reported the large value of third order non-linear susceptibility ( 3.4 10 11 esu) for CdS NCs in polymeric film. In the present work, the simple processing technique has been used for the synthesis of CdS/PS nanocomposite. PVP has been used as a steric stabilizer during synthesis. The structural and optical properties of the nanocomposite have been studied with X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), UV–vis absorption and Photoluminescence (PL) spectroscopy. The non-linear optical properties of the CdS/PS nanocomposite have been studied using Z-scan technique.
2. Experimental details 2.1. Chemicals used Cadmium acetate (Cd(CH3COO)2) and benzene were purchased from Qualigens fine chemicals, Sodium sulfide (Anhydrous purified) and Ammonia were purchased from Merck Specialities Pvt. Ltd., Polyvinyl pyrrolidone (PVP) and Polystyrene (PS) were obtained from Sigma Aldrich. Deionized water was used in all experiments and all of the other chemicals were of analytical grade and used as purchased without any further purification. 2.2. Synthesis of CdS/PS nanocomposite CdS/PS nanocomposite has been prepared by chemical route using ex-situ technique. It involves two steps: (1) Preparation of CdS NPs (2) Dispersion of CdS NPs into PS matrix. Cd(CH3COO)2 and sodium sulfide were used as a cadmium source and sulfur source, respectively. Cadmium source solution was prepared in 30 ml deionized water by dissolving 2.8 g of Cd(CH3COO)2 to it. Sulfur source was prepared separately in 30 ml deionized water by dissolving 820 mg of sodium sulfide to it. CdS NPs were generated by dropwise addition of sodium sulfide solution to cadmium acetate solution under continuous stirring. The pH of the solution was adjusted to 11 by dropwise addition of aqueous ammonia. After 5 min of stirring, 0.5 g of PVP was added and solution was left for 2 h for stirring. The prepared CdS NPs were collected, dried and crushed to obtain the fine powder. Yellow colored CdS NPs were obtained after crushing. Polystyrene solution was prepared by adding 3 g of polystyrene to 100 ml of benezene. For the preparation of CdS/PS nanocomposite, 500 mg of CdS NPs were redissolved in 10 ml of benzene. CdS nanoparticle solution was added dropwise to the 50 ml polystyrene solution with continuous stirring at room temperature to obtain CdS/PS nanocomposite. For XRD analysis, the thin films of the CdS/PS nanocomposite were deposited on glass substrate by solution casting method at room temperature.
2.3. Characterization XRD patterns were recorded by using Phillips PW-1710 X-ray diffractometer with CuKα radiation in the 2θ range from 5° to 90°. FTIR spectra of the samples were recorded using PerkinElmer PERX 1 FTIR spectrophotometer with spectral resolution 1 cm 1. UV absorption spectra were recorded using single beam Spectroscan 30 spectrometer in the wavelength range of 300–800 nm. Photoluminescence properties of the samples were recorded on a Shimadzu Spectrofluorophotometer RF-5301PC. The non-linear optical properties of the CdS/PS nanocomposite were studied by using Z-scan technique. A continuous wave (CW) He–Ne laser operating at a wavelength of 632.8 nm was used to perform open and closed aperture Z-scan experiment. During Z scan measurements, the sample was translated along the z-direction (in the direction of wave propagation) around the focal point by keeping the wavelength of laser light fixed. The non-linear absorption coefficient (β) and the imaginary part of the third order non-linear (3) ) were obtained from the open aperture Z-scan susceptibility ( χIm measurements. In case of closed aperture Z-scan scheme, the magnitude of peak-valley transmission gives the measure of nonlinear refractive index (n2) and the real part of third order nonlinear susceptibility ( χR(3) ). To perform intensity dependent Z-scan measurements, a set of polarizer and analyzer was used in front of laser source. All the Z-scan measurements were performed at room temperature.
3. Results and discussion 3.1. X-ray diffraction Fig. 1 shows the XRD pattern of PS and CdS/PS NC thin films, where the scattered intensity has been plotted as a function of diffraction angle (2θ) in the range of 5–90°. XRD pattern of PS shows a broad peak at 2θ ¼19.27° with a small hump at 2θ ¼10.29° indicating the amorphous nature of polymer [25]. With the addition of CdS NPs into the PS matrix, the crystalline nature of the nanocomposite has been obtained. XRD pattern of CdS/PS NC exhibit prominent broad peaks centered at 2θ ¼26.46°, 43.59° and 51.04°. These peaks correspond to the cubic phase of CdS with diffraction planes [111], [220] and [311] respectively [26], [JCPDS file no. 00-10-454]. The weak intensity peak at 2θ ¼33.25° indicates the [200] plane of cubic CdS [JCPDS file no. 00-10-454]. The lattice parameter ‘a’ has been determined by using relation [27];
1 (h2 + k 2 + l2) = 2 d a2
Fig. 1. XRD spectra of PS and CdS/PS nanocomposite.
(1)
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where h, k and l represent the lattice planes. The value of ‘a’ calculated from the ‘d’’ value corresponding to [111] plane is 5.827 Å. The crystallite size has been calculated by using Debye Scherrer's formula [28];
D=
kλ β cos (θ )
(2)
where β is the full width at half maxima (FWHM), θ is the diffraction angle, λ is the X-ray wavelength and k is Scherrer's constant of the order of unity. The diffraction peaks are considerably broad indicating a very fine size of the NPs. The diffraction peak broadening may be due to the inhomogeneous strain present in the films [29].The value of strain (ɛ) has been calculated by using following relation [30];
ε=
β 4 tan (θ )
(3)
Dislocation density (δ) which is a measure of total length of dislocation per unit volume of the crystal and has been determined by using following formula [31];
δ=
n D2
(4)
here factor n is equal to unity, giving the minimum density of dislocation. The values of 2θ, crystallite size, lattice constant, strain and dislocation density corresponding to each plane are given in Table 1. The average crystallite size and average strain calculated by using Eqs. (2) and (3) is 2.54 nm and 7.32 10 1 respectively. This is due to the structural disorderness which occurs during the thin film formation, when the adjacent grains come into contact. Also, the thermal expansion coefficient differences between the substrate and thin film may contribute to the strain [32]. 3.2. Fourier transform infrared spectroscopy FTIR spectra of the PS and CdS/PS nanocomposite are shown in Fig. 2. The various functional groups present in the PS and CdS/PS nanocomposite corresponding to their wavenumber are shown in Table 2. The presence of typical styrene bands at 665.29 cm 1, 701.20 cm 1, 759.63 cm 1, 1034.76 cm 1, 1175.94 cm 1 and 3034.95 cm 1 is due to the polystyrene in the nanocomposite [33]. 1175.94 cm 1, The absorption bands at 701.20 cm 1, 1 1 and 3070.81 cm can be assigned to the asym1526.35 cm metric and symmetric stretching vibrations of ¼C–H, C–O, C ¼ C and C–H groups respectively. The characteristic vibrational absorption peak for the Cd–S bond has been observed at 406.57 cm 1 in CdS/PS nanocomposite, but the intensity of this peak is weak [34]. 3.3. Absorption spectroscopy Fig. 3(a) shows the absorption spectra of PS and CdS/PS nanocomposite. PS shows the absorbance only in the UV region but with the addition of CdS NPs the absorbance shifted to visible region. CdS/PS nanocomposite shows an absorption peak at 441 nm which is due to the optical transition of Ist excitonic state in the CdS NPs [35]. The corresponding band gap of the CdS/PS Table 1 Crystallite size, strain and dislocation density calculated from XRD data. 2 θ (deg)
hkl
D (nm)
ε
δ ( 1018)
a (Å)
26.46 43.59 51.04
111 220 311
5.28 1.63 0.72
0.03 0.06 0.12
0.04 0.38 1.93
5.83 5.86 5.93
57
nanocomposite has been calculated by using relation Eg (eV) ¼ (1238/λ (nm)). The band gap of the CdS/PS nanocomposite calculated from above relation is 2.81 eV, whereas the bulk band gap of CdS is 2.42 eV. The increase in the band gap value of CdS/PS nanocomposite confirms the Quantum confinement effect. As we know that the Quantum confinement is responsible for the increase of energy difference between the energy states and band gap. PS does not show any absorption peak, therefore its band gap value has been calculated by using Tauc's relation which relates the absorption coefficient (α) to band gap as;
(αhν )1/ n = A (hν − Eg )
(5)
where Eg is the optical band gap energy, ν is the frequency, h is Planck's constant, A is a constant, and n is the constant parameter which is equal to 1/2 for direct allowed transition, 2 for indirect allowed transition, 3/2 for direct forbidden transition and 3 for indirect forbidden transition. Using Tauc's relation the band gap value calculated for PS is 3.93 eV as shown in Fig. 3(b). The size of the NPs has been calculated from the band gap information by using effective mass approximation (EMA) method [36] given by following formula;
⎛ ℏ2π 2 ⎞ ⎛ 1 1 ⎞ 1.8e2 ⎟⎜ ⎟− Egn = Egb + ⎜ + 2 * εR mh* ⎠ ⎝ 2R ⎠ ⎝ m e
(6)
where Egb is the bulk band gap value, Egn is the nanoparticle band gap value taken from absorption data, me* is the effective electron mass, mh* is the effective hole mass and ε is the dielectric constant of the medium. The second term in Eq. (6) represents the quantum localization energy whereas the third term corresponds to the Coulomb potential energy, respectively [36]. The coulomb term for electron–hole interaction is very small as compared to the Quantum confinement term, thus usually neglected. For CdS, me* = 0.21me and mh* = 0.80me where me is the free electron mass [36]. The value of particle radius (R) calculated from Eq. (6) is 2.41 nm (particle diameter, 2R ¼4.82 nm), which is smaller than bulk exciton radius (3 nm) [37] which indicates the strong confinement in the CdS NPs. 3.4. Photoluminescence properties PL spectra of the samples have been recorded at an excitation wavelength of 320 nm and shown in Fig. 4. PL spectra of PS shows the emission peak around 363 nm. This is due to the generation of luminescent substances such as benzoic acid, styrene, methybenzonate, double bond in the main chain, stilbene, etc. These are produced by reaction of oxygen with radicals, formed from the elimination of hydrogen atom from the carbon adjacent to the phenyl group of PS under light illumination [38]. In the PL spectra of CdS/PS nanocomposite, several emission bands has been observed at 430 nm (2.87 eV), 473 nm (2.62 eV) and 563 nm (2.19 eV). Generally in semiconductor nanomaterials, the various mechanisms contributing to the PL emission are: (a) band edge emission, (b) deep trap emission, (c) surface defect emission and (d) the nanoparticle size distribution [39]. The emission band at 2.87 eV is in the vicinity of calculated band gap from absorbance spectra and is due to the band to band transition. In CdS NPs, the four types of point defects: interstitial cadmium (ICd), interstitial sulfur (IS), cadmium vacancy (VCd) and sulfur vacancy (VS) exists which play an important role in photoluminescence spectra [40]. The green and yellow emission bands at 473 nm and 563 nm are due to the interstitial sulfur and interstitial cadmium defect states respectively [41]. Fig. 5 shows the schematic representation of different transitions involved in PL spectra. In case of CdS/PS nanocomposite, the CdS NPs plus the luminescent substances in PS matrix are contributing to the PL emission.
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Fig. 2. FTIR spectra for PS and CdS/PS nanocomposite.
Fig. 4. Photoluminescence spectra of PS and CdS/PS nanocomposite excited at an excitation wavelength of 320 nm.
Table 2 Assignment of functional groups for FTIR spectra. Wave number (cm 1)
Functional group
CdS/PS NC
PS
406.57 665.29 701.20 759.63 846.44 1034.76 1175.94 1393.69 1477.77 1526.35 1814.82 1959.82 3034.95
– 665.39 700.83 759.13 846.28 1034.83 1176.63 1393.11 1477.79 1526.14 1814.74 1960.05 3034.96
Cd–S Phenyl group ¼ C–H ¼ C–H ¼ C–H C–O C–O –C–H C ¼C C ¼C C ¼O C ¼C C–H
3.5. Non-linear optical properties of CdS/PS nanocomposite The third order non-linear optical properties of the CdS/PS nanocomposite have been investigated by Z scan technique. The simplicity of the Z scan technique is that the non-linear parameters such as n2, β and third order non-linear susceptibility (χ(3))
Fig. 5. Schematic representation of various transitions involved in photoluminescence spectra.
can be obtained from the single experiment. Fig. 6(a) and (b) shows the open aperture Z scan curve and the variation of q0(zs) with sample position, where q0(zs) is the parameter determining the strength of the non-linearity. The value of q0(zs) has been determined from the normalized transmittance T (zs ) (OA Z scan) by using following relations which are valid for T (zs ) Z0.224 and T (zs ) Z0.243 for Gaussian and hyperbolic secant pulses respectively:
Fig. 3. (a) UV–vis spectra of PS and CdS/PS nanocomposite and (b) variation of (αhv)2 vs. hv for PS.
S.K. Tripathi et al. / Optics Communications 352 (2015) 55–62
59
Fig. 6. (a) Open aperture Z-scan curve for CdS/PS nanocomposite and (b) variation of q0(zs) with sample position.
⎧ 2 3 ⎪a 0 + a1T (zs ) + a 2 T (zs ) + a 3 T (zs ) for T (zs ) ≤ 0.75 q0 (zs ) = ⎨ ⎪ c 2 c0 + c1 [T (zs )] for T (zs ) ≥ 0.75 ⎩
(7)
here the coefficients a0, a1, a2, a3, c0, c1, c2 have values 15.66, 37.45, 30.76, 8.97, 2.301, 2.156, 1.563 for Gaussian pulses. As seen from the open aperture Z-scan (Fig. 6(a)), the CdS/PS nanocomposite exhibits reverse saturable absorption. With increasing laser beam intensity, the transmittance of the nanocomposite increases. The value of β has been calculated from open aperture Z scan curve by using following relation [42];
λzR Q 0 β= 2P0 Leff
(8)
Leff = (1 − exp ( − αL ))/α
(9)
here L is the sample length and α is the linear absorption coefficient. The value of Q0 and zR has been calculated from Fig. 6(b). The maximum value of q0(zs) at z ¼z0 gives the value of Q0 whereas from the FWHM of this graph, the value of zR has been calculated, as at zs = z0 ± zR , the value of q0(zs) is equal to Q0/2. Fig. 7 shows the closed aperture Z scan curves for CdS/PS nanocomposite with a signature of peak followed by a valley indicating the negative type of non-linearity in the nanocomposite. From closed aperture measurements, the value of n2 has been calculated by using following relation [43];
Δφ0 λ
2πI0 Leff
(10)
where |Δφ0 | represents the phase change of the laser beam due to non-linear refraction and Io is the input irradiance of laser light at focal point. The value of |Δφ0 | has been calculated from the difference between the normalized peak and valley transmittances (ΔTp v) by using following relation [44];
Δφo =
χ (3) =
(3) 2 (χR(3) )2 + (χIm )
where,
χR(3)
is the real part of
(12) χ (3)
and
(3) is χIm
the imaginary part of
χ (3) and given as;
where λ is the laser wavelength (632.8 nm), P0 is the peak power of the pulses and Leff is the effective thickness of the sample, given by the relation
n2 =
The non-linear behavior of the CdS/PS nanocomposite is due to the CdS NPs as the pure PS does not show any non-linear behavior [22]. The values of β and n2 calculated for CdS/PS nanocomposite using Eqs. (8) and (10) are given in Table 3 and are in good agreement with the literature results [45]. Furthermore, the value of χ(3) has been determined from β and n2 values by using following relations;
ΔTp − v
0.406 (1 − S )0.25
Here S ( 0.39) is the linear transmittance of the aperture.
(11)
χR(3) = 2n2 n2ε 0 c
(13)
(3) χIm = ε 0 n2cλβ/(3π )
(14)
here n is the linear refractive index of the sample (2.213), ε0 is the permittivity of free space (8.854 10 14 F/cm) and c is the velocity (3) calculated using Eqs. (13) and (14) of light. The value of χR(3) and χIm 13 18 is 1.6 10 and 3.5 10 m2/V2, respectively. Table 4 shows (3) ) for CdS/PS the comparison of non-linear parameters (β, n2, χIm nanocomposite with the literature results. It has been found that the value of n2 is comparable to the value reported by Rao et al. [45] for TGA (thioglycerol) capped CdS NPs with size 4.5 nm. But this value is much smaller than the value reported by Chin et al. [22] for CdS/PS nanocomposite by approximately 5 orders. The large difference in optical non-linearity in CdS/PS nanocomposite is due to the Quantum confinement effect as observed from the absorption spectra (Fig. 3(a)). In nanosize regime, due to the high surface area activity and the special surface structure leads to the change in optical properties with the surrounding environment [42,50]. Under laser light of wavelength 632.8 nm, the enhancement of local electric field inside the NPs contribute to the optical non-linear behavior of the nanocomposite. Mamta et al. [42] had reported that the surface states present in the nanocomposite and the dielectric confinement effect also contributes to the large optical non-linearity. In a particular experiment, the various physical mechanisms contributing to the non-linear refractive index are electronic, vibrational, electrostriction and thermal, depending on the wide range of time scales as;
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Fig. 8. Variation of n2 with laser beam intensity.
Fig. 7. Closed aperture Z-scan curve for CdS/PS nanocomposite. Table 3 Variation of non-linear parameters and thermo optical coefficient with laser beam intensity. I0 ( 102 W/cm2)
β ( 10 8 cm/W)
n2 ( 10 8 cm2/W)
(3) ( 10 18 m2/V2) χIm
dn/dT ( 10 8 K 1)
0.82 1.02 1.59 3.98
40.1 34.1 21.2 4.8
14.5 11.9 8.6 3.9
3.5 2.9 1.8 0.42
1.81 1.48 1.06 0.48
n2 = n2 (electronic) + n2 (vibrational) + n2 (electrostriction) + n2 (thermal)
(15)
The time response for the electronic, vibrational and electrostrictive contribution is 10 15, 10 13 and 10 8 s respectively [51]. The contribution of thermal effects is at much longer time scales (tens of μs). In case of nanosecond laser pulses, only the first two mechanisms contribute to the non-linear properties. In the present work, CW He–Ne laser is used, thus the main contribution to the non-linearity of the nanocomposite is due to the thermal effects [52]. Due to the temperature gradient effect produced by thermal heating, the refractive index change occurs across the sample resulting in the lensing effect in the sample [53]. The thermal effect changes depend on the laser beam intensity. To confirm the presence of thermal effects, the intensity dependent Z-scan measurements have been performed (Figs. 6 and 7). The thermo optical coefficient (dn/dT) has been calculated by using following relation [23]:
dn λκ =− Δφo dT Pαo Leff
(16)
where κ is the thermal conductivity (0.09 W cm 1 K 1), Δφo is the
on-axis phase shift, P is laser power. With increasing laser beam intensity, the transmittance and the difference between Tp and Tv increases resulting in increase in n2 value as shown in Fig. 8. With increasing laser beam intensity, the non-linear refractive index and thermo optical coefficient increases indicating the presence of thermo optical effects. The values calculated are given in Table 3. Also the separation between the peak-valley increases with increasing laser beam intensity indicating the non Kerr type non-linearity and the presence of the thermal component in the non-linear behavior of CdS/PS nanocomposite [53]. Thus, the combination of Quantum confinement effect and the thermal effect results in large non-linear optical response of CdS/PS nanocomposite. The enhanced UV–vis absorption and PL intensity makes CdS/PS nanocomposite suitable for electroluminescent devices. The large value of χ 3is responsible for its use in various non-linear optical devices.
4. Conclusion CdS/PS nanocomposite has been synthesized by ex-situ technique using benzene solvent. The cubic phase of CdS NPs has been confirmed from the XRD spectra with average crystallite size
Table 4 (3) Comparison of non-linear optical parameters (β, n2 and χIm ) with the literature.
Sample (Size)
Size (nm)
Wavelength (nm)
β (cm/W)
n2 (cm2/W)
(3) (m2/V2) χIm
Reference
CdS/PS NC (I0 ¼3.98 102 W/cm2) Bulk CdS CdS NPs CdS NPs CdS/PS NC CdS NPs CdS NPs CdS/Sodium borosilicate
4.8 – 4.5 5–7 3–6 3.3 3.0 10–20
632.8 532 532 532 532 530 532 770
4.8 10 8 5.4 10 9 3.1 10 8 7.6 10 10 – – 6.0 10 11 6.3 10 9
3.9 10 8 5.3 10 13 6.9 10 8 – 1.0 10 13 3.0 10 14 – 2.2 10 12
4.2 10 19 – – 2.9 10 22 – – 2.6 10 22 1.7 10 18
Present work [20] [45] [46] [22] [47] [48] [49]
S.K. Tripathi et al. / Optics Communications 352 (2015) 55–62
2.54 nm. In FTIR spectra, the characteristic vibrational band for Cd–S bond at 406.57 cm 1 has been observed along the typical styrene bands. The absorption region of PS has been found to extend from UV region to visible region with the addition of CdS NPs to it. Photoluminescence spectra of CdS/PS nanocomposite consists of band to band transition emission plus the green and yellow emission bands due to the defect states present in it. CdS/ PS nanocomposite shows good non-linear optical properties with β and n2 values 4.8 10 8 cm/W and 3.9 10 8 cm2/W respectively. CdS/PS nanocomposite synthesized by simple processing technique with good optoelectronic properties may be useful for the fabrication of novel optical sensors and electronic devices.
Acknowlegements This work is financially supported by University Grant Commission (UGC) (Major Research Project) [F. No. 42-781/2013(SR)], N. Delhi. Ms. Ramneek Kaur is thankful to UGC, N. Delhi for providing the fellowship.
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