Study on the Third-Order Nonlinear Optical Properties of Cd2+ Ion Doped Zns/PVP Nanocomposite FilmsB. Suresh1, S. Ramachandran1 and G. Shanmugam2

Cd 2+ ion doped ZnS/PVP nanocomposite films have been prepared using in-situ chemical method. XRD studies confirm the cubic structure of ZnS for all the films and the nanocrystallite size of ZnS is found to varying from 3.82 to 4.07 nm with Cd doping. The morphology of the films has been analyzed using SEM micrograph which shows the mono dispersion of ZnS nanoparticles in PVP matrix. The interaction between ZnS nanoparticles and PVP polymer matrix has been discussed using FTIR spectra. Optical absorption spectra of the films reveal the blue shift on the absorbance onset with the comparison of bulk ZnS and the estimated optical band gap energy decreases with increasing the concentration of Cd dopant. Photoluminescence spectra show a blue emission peak for all the films. The Z-scan results of three samples show a reverse saturation absorption process in open Z-scan experiment and self-focusing behavior in closed Z-scan experiment. Both ZnS/PVP and Cd 2+ ion doped ZnS/PVP films exhibited the large optical nonlinearity and the value of nonlinear refractive index (n 2 ), nonlinear absorption coefficient (  ), third-order nonlinear optical susceptibility (  (3) ) and figure of merit (FOM) increase with Cd doping. The estimated n 2 ,  ,  (3) and FOM values were found to be high for 6 mol% Cd doped ZnS/PVP nanocomposite film and they are about 1.771  10 -10 m 2 W − 1 , 4.148  10 -3 mW − 1 , 1.603  10 -4 esu and 9.576  10 -6 esu m respectively. The experimental results clearly showed that the Cd 2+ ion doped ZnS/PVP nanocomposite film is a worthy candidate for the future nonlinear optical device fabrication.


Introduction
In recent years, semiconducting nanoparticles have been widely investigated by many researchers due to the quantum confinement effect and notable changes in the chemical and physical properties of these particles with the comparison of those of bulk solids. They doped with different metal ions produce new door for optical studies and potential applications in optical device fabrication. Among the semiconductors, ZnS is a well known II-VI group semiconductor with a energy band gap of 3.6 eV and is widely used as a phosphor and in luminescent device [1][2][3][4][5].
Transition metal ions doped ZnS nanoparticles have received much attention due to their exceptional luminescent properties. During the past decades, luminescent properties have been extensively investigated on Mn doped ZnS [1], Cu doped ZnS [2], Ag doped ZnS [3], Ce doped ZnS [4] and Pb doped ZnS nanoparticles [5]. Large third-order optical nonlinearity can be expected from ZnS nanoparticles due to its exceptional luminescent properties. Initially, third-order nonlinear optical studies have been carried out on ZnS nanoparticles dispersed in solution [6,7]. Later, this study has been carried out on ZnS thin films using different substrates due to the instability of ZnS nanoparticles in solution. During the Z-scan measurements on substrate supported ZnS thin films, it was very difficult to determine the accurate nonlinear optical coefficients of ZnS nanoparticles due to the contribution of optical nonlinearity of substrates. In order to resolve it, different substrates were used for making ZnS thin films to extract the true values of nonlinear optical coefficients of ZnS nanoparticles [8][9][10][11]. In recent years, self standing nanocomposite films containing semiconducting nanoparticles embedded in polymer matrix have been widely attracted by many researchers due to the following factors: (a) mono-dispersion of nanoparticles can be easily achieved in polymer matrix, (b) true nonlinear optical coefficients of nanoparticles can be obtained, (c) easy preparation and estimation of nonlinear optical coefficients, (d) improved thermal and mechanical stabilities [12][13][14][15][16].
In this paper, third-order nonlinear optical properties of Cd doped ZnS/poly vinyl pyrrolidone (Cd:ZnS/PVP) nanocomposite films have been reported. PVP was chosen as polymer matrix due to its outstanding film forming property, biocompatible and biodegradable polymer, non-toxic, high solubility in water, good mechanical strength, high thermal stability, high transparency (from ultraviolet to near infrared regions) and easy processability [16][17][18][19].
To our best of our knowledge, this is the first report on nonlinear optical properties of Cd doped ZnS/PVP nanocomposite films using Z-scan technique. During the Z-scan measurements, large value of third-order optical nonlinear coefficients was obtained. They are about two orders of magnitude larger than that of ZnS nanoparticles dispersed in solution and ZnS thin films using different substrates.

Preparation of Cd doped ZnS/PVP nanocomposite films
For synthesis of nanocomposite films, 1 g of PVP polymer dissolved in double distilled water was constantly stirred using magnetic stirrer until the solution turns into transparent and colorless. Zinc acetate dihydrate and sodium sulfide with concentrations of the molar ratio of 1:1 were fixed for preparation of all the films. An aqueous solution of zinc nitrate (solution A) and cadmium nitrate (solution B) in double distilled water was prepared at room temperature. Then solutions (A and B) were stirred vigorously for 20 min using magnetic stirrer until the mixture solution turns into transparent solution. The mixture solution of zinc nitrate and cadmium nitrate was added into the PVP solution. Then sodium sulfide aqueous solution was slowly mixed with above solution with constant stirring until the whole solution becomes to milky white color, representing the initial formation of ZnS. The stirred solution was cast on the glass slide and then dried at 70C for 1 h in vacuum oven to evaporate the solvent contents from the Cd doped ZnS/PVP nanocomposite films. Similarly, 0, 4 and 8 mol% Cd doped ZnS/PVP nanocomposite films were synthesized. The film thickness of all the films was in the range of 65-125µm.

Characterization
The structural studies were carried by using a Rigaku X-ray diffractometer with CuKα radiation (λ = 1.54 Å) in the range of 20-60. The surface morphologies were carried out by using JEOL Model JSM-6390LV scanning electron microscope. The FTIR spectra of three films were recorded using a PerkinElmer Spectrum GX FTIR spectrophotometer in the wavenumber range of 4000-400 cm -1 . The room temperature optical absorption spectra were recorded using a PerkinElmer Lambda 35 spectrometer in the range of wavelength from 250 to 1000 nm.  .8 corresponding to hkl planes of (111), (220) and (311) could be assigned to cubic structure of ZnS (JCPDS card no: 05-566). From the XRD patterns of Cd doped ZnS/PVP films, no additional peaks were found and it confirms the purity of Cd doped films. It can be seen in Fig. 1, the position of diffraction patterns shifted towards lower angle side with Cd doping in ZnS/PVP which is due to the fact that the ionic radius of Cd 2+ (0.95 Å) is larger than the Zn 2+ (0.74Å), i.e. the doping of Cd in the Zn site and the change in the ionic radius will make the variation in interpalanar distance (d) induce this type of angle shift [4]. The lattice constant (a) of three films can be calculated by using the relation, d = a/(h 2 +k 2 +l 2 ) 1/2 where d is the interplanar spacing and the values for undoped, 3 and 6 mol% Cd doped films are 5.422 Å, 5.416Å and 5.11Å respectively. The lattice constant value decreases with increasing Cd doping.
The average crystallite size of all the films can be calculated using the Debye-Scherrer formula: where D is the average crystallite size, λ is the wavelength of the X-rays, β is the full width at half maximum and θ is the diffraction angle [4,12]. The nanocrystallite size is found to be 3.82 nm for ZnS/PVP, 3.97 nm for 3 mol% Cd doped ZnS/PVP and 4.07 nm for 6 mol% Cd doped ZnS/PVP nanocomposite films.

SEM analysis
The SEM images of (a

UV-Vis optical absorption studies
where α is the absorption coefficient, h is the incident photon energy, Eg is the band gap of ZnS nanoparticles and A is a constant [12].
where me and mh are electron and hole effective masses and R is the particle size [20].

Photoluminscence studies
The PL spectra of (a

Z-scan results
Open aperture Z-scan plot of all the nanocomposite films is shown in Fig. 7(a). The open aperture (OA) plot shows a normalized transmittance valley which represents reverse saturable absorption behavior with positive nonlinear absorption. A closed aperture Z-scan plot of all the nanocomposite films is shown in Fig. 7(b). The closed aperture (CA) plot shows a valley to peak nature which exhibits strong self-focusing with positive nonlinear refraction [12][13][14][15][16]. The data were analysed using the procedures described by Sheik-Bahae et al [22]. The nonlinear refractive index n2 (esu) can be calculated using the following equation:     The calculated nonlinear absorption coefficients are found to be 1.47010 -3 , 2.94610 -3 and 4.14810 -3 mW −1 for ZnS/PVP, 3 and 6 mol% Cd doped ZnS/PVP nanocomposite films, respectively. The estimated  values are two orders of magnitude larger than that of the Co doped ZnS thin film [23]. The real (Reχ (3) , esu) and imaginary (Imχ (3) , esu) parts of the third-order nonlinear susceptibility can be estimated using the following equations: where ω is the angular frequency of light field. The real part of the third-order nonlinear susceptibility, Reχ (3) , is about 6.28010 -5 , 8.94110 -5 and 1.03710 -4 esu for ZnS/PVP, 3 and 6 mol% Cd doped ZnS/PVP nanocomposite films respectively whereas the imaginary part of the third-order nonlinear susceptibility, Imχ (3) , is about 5.70910 -5 , 9.88710 -5 and 1.22210 -4 esu for ZnS/PVP, 3 and 6 mol% Cd doped ZnS/PVP nanocomposite films respectively.
The absolute value, χ (3) , of third-order nonlinear optical susceptibility of all the nanocomposite films can be estimated using the following equation: The value of χ (3) is found to be 8.48810 -5 , 1.33310 -4 and 1.60310 -4 esu for ZnS/PVP, 3 mol% Cd doped ZnS/PVP and 6 mol% Cd doped ZnS/PVP nanocomposite films respectively. The χ (3) values are four orders of magnitude larger than the reported value of some other representative nonlinear optical material cobalt phthalocyanine multilayer films [25] and N-doped graphene oxide nanocomposites [26].  [24]. The large value of nonlinear optical parameters obtained in this present study strongly recommends that the Cd 2+ doped ZnS/PVP self-standing nanocomposite films are promising candidate for the fabrication of future nonlinear optical devices.

Conclusions
ZnS to self-focusing. The present report strongly recommends that the Cd doped ZnS/PVP nanocomposite film is a promising material for applications in the fabrication of nonlinear optical devices.

Conflict of Interest
The authors declare that they have no conflict of interest