Tuning the non-linear optical absorption properties of Eu3+-doped NiWO4 nanostructures

NiWO4 nanostructures doped with different ratios of Eu3+ have been prepared by a chemical precipitation method. The influence of Eu3+ on NiWO4 nanostructures were characterized using X-ray diffraction (XRD), UV–visible diffuse reflectance spectra, scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Raman. XRD patterns display that the samples crystallize to monoclinic wolframite structure. SEM images reveal that the particles are well uniformly dispersed with average particle size lies around 40–50 nm. Third-order nonlinear optical properties were studied by a Z-scan technique at 532 nm using continuous wave diode pumped Nd:YAG laser. Open and closed aperture results reveal the nanostructures to possess reverse saturation absorption and negative nonlinear refraction. The calculated absorption coefficients (β), imaginary part of third-order susceptibilities Imχ(3) are in the order of 10–6(cm/W) and 10–7 esu. These results indicated that the synthesized nanostructures could be promising materials for optical device applications.


Introduction
The third-order nonlinear optical (NLO) properties of nanostructured materials have become a frontier area for researchers to fabricate devices for various optical applications in areas such as photonics, telecommunications, optical switching, sensors, optical limiting. [1,2]. Although there are many competing materials qualifying for NLO device applications but researchers are still finding for the easy synthesizable materials with enhanced optical nonlinearity. In this regard, metal tungstates with formula MWO 4 are most interesting and important inorganic materials have received attention due to their attractive size and structure-dependent optical and electronic properties [3]. Such materials have found applications in scintillation counters, optical fibers and lasers [4]. Nickel tungstate (NiWO 4 ) is one of the bivalent transition metal tungstate having quite interesting properties serve as an excellent candidate for applications in microwave devices [5], photocatalyst [6], etc. It is quite interesting that tungstate compounds with relatively large bivalent cations (such as, Ca, Ba, Pb and Sr) whose ionic radius larger than 0.099 nm can exist in the tetragonal scheelite structure and when coordinated with bivalent cations having smaller ionic radius (such as, Ni, Mn, Fe and Mg) less than 0.077 nm exhibit monoclinic wolframite structure [7]. It is reported that crystals containing W 6? and cations with non-bonded electron pairs have shown to possess second order NLO behavior [8,9]. NLO studies PbWO 4 with scheelite structure and ZnWO 4 with a wolframite structure have reported to exhibit two photon interband absorption and two photon induced one photon absorption [10]. It is well-known that the optical properties of the host are dependent on the particle size and this can be tuned by anchoring with a suitable dopant. Modification of the desired properties could be achieved through application of doping method with certain rare-earth elements [11]. Rare-earth-based materials have gained significant attention on account of their excellent optical properties inherited from intra 4f transitions. Europium as rare-earth cations have several advantages due to its non-degenerate ground, emitting states and 5 D 0 -7 F 0 transition could be used as applications in luminescent devices [12]. Europium-doped metal tungstates will exhibit a strong absorption at UV region as well as near blue region due to f-f transition of Eu 3? ions. It is reported that tungstates can effectively transfer energy to trivalent rare-earth ions-like europium [13]. Doping of such materials is expected to dramatically increase the optical nonlinearity of the host. Numerous methods have been employed such as co-precipitation [14], solvothermal [15], modified citrate complex technique [16], microwave irradiation [17], hydrothermal [18] for the synthesis of NiWO 4 nanostructures.
In the present work, we explore the third-order nonlinear optical properties of Eu-doped NiWO 4 nanostructures synthesized by chemical precipitation method. Both open and closed aperture measurements were carried out using a continuous wave Nd:YAG laser at 532 nm and the data were extracted. Experimental results have shown to possess excellent NLO behavior, making them suitable for various optical applications in continuous wave domain.

Synthesis of NiWO 4 nanostructures
NiWO 4 nanostructures were synthesized by chemical precipitation technique. In this method 0.5 M of Ni(NO 3 ) 2 6H 2 O solution was slowly dropped into 0.5 M of Na 2 WO 4 .2H 2 O at 6 ml/min of dropping rate under vigorous stirring. Slowly the color of the solution was turned into pale green, the indication of the formation of nickel tungstate nanoparticles. Then the solution was centrifuged at 10,000 rpm for 10minutes for several times in order to ensure the complete removal of byproduct. At last the sample was washed with absolute ethanol and dried in hot air oven at 353 K for 30 min. Then the sample was annealed in furnace at 873 K for three hours with the heating rate of 2°C/min. For europium doping different concentration of Eu(NO 3 ) 3 Á6H 2 O (0.3, 0.5 and 0.7 wt%) was added with respect to Ni 2? ion concentration and same procedure was followed as mentioned above without any change. Finally, the annealed sample was grained into fine powder and preserved for further studies.

Characterization
The X-ray diffraction patterns were recorded on a PANAalytical X'pert PRO X-ray diffractometer using Cu Ka radiation as the X-ray source. UV-vis absorption spectra were obtained using Perkin Elmer Lambda 35 spectrophotometer. SEM images were obtained on FEI Quanta FEG 200-High Resolution Scanning Electron Microscope. Transmission Electron Microscope (TEM) micrographs were acquired with a JEOL JEM-2100 high-resolution TEM (HR-TEM). The optical nonlinearity of the samples was studied by Zscan technique, using diode pumped Nd: YAG laser (532 nm, 50 mW) as excitation source.

Structural analysis
The XRD pattern of pure and Eu 3? -doped NiWO 4 nanostructures are shown in Fig. 1. For pure NiWO 4 nanostructures the diffraction peaks originated at (010), (100), (011), (110), (-111), (020), (002), (-102), (-202), (-113) and (-132) planes are indexed to monoclinic wolframite phase with space group P 2/c in accordance with JCPDS data card no: 00-ss015-0755 [19]. No impurity peaks can be seen which confirms the synthesized samples are pure and crystalline in nature. No additional peaks of other phases related to pure oxide phase such as Eu 2 O 3 or NiO phase is seen, implying the formation of homogenous pure phase of Eu-doped NiWO 4 . This shows that Eu doping does not induce any significant change in phase during synthesis and have entered the host lattice without affecting the crystal structure. It is observed that the addition of Eu 3? dopant ions decreases the intensity of the diffraction peaks of NiWO 4 indicating the substitution of Eu 3? (1.06Ȧ ) causes lattice distortion into Ni 2? (0.7 Ȧ ). As the doping concentration increases, the incorporation of Eu into the crystal lattice increases and causes lattice distortion due to its large size. The average crystallite size calculated from Scherrer formula was found to be 58.2, 55.7, 51.5 and 48.3 for pure, 0.3, 0.5 & 0.7% Eu 3? -doped NiWO 4 nanostructures.

Morphological studies
The morphology and particle size distribution of pure and Europium-doped NiWO 4 nanostructures are shown in Fig. 2. The morphology of pure NiWO 4 nanostructures shows cubical shape with an average particle size between 50 and 60 nm. It is quite interesting to note that the diameter of the particle decreases as doping concentration increases with similar morphology. The decrease in particle size with increase in dopant concentration is validated through statistical data. This indicates doping of Eu does not influence the surface morphology rather restricts the growth of crystal at higher doping concentration.
The TEM micrographs of the synthesized nanostructures were taken to examine the morphology and precise determination of size. It can be seen from the Fig. 3, the nanoparticles exhibit relatively uniform, well-dispersed morphology with narrow size distribution of particles with size around 50-60 nm, which is close to that calculated from XRD.
The elemental compositions of pure and Eu 3?doped NiWO 4 nanostructures are quantitatively analyzed using energy dispersive X-ray spectroscopy (EDX) analysis measurement and are shown in Fig. 4. The product consists of Ni, W, Eu and O elements with appropriate proportion, demonstrating the controllability in chemical composition. Neither N nor C signals were detected in the spectra, indicating the product is pure and free of impurity. The relative element content of Eu 3? -doped NiWO 4 nanostructures are shown on Table 1.

Diffused reflectance UV spectroscopy
The optical absorption spectra of NiWO 4 nanostructures are shown in Fig. 5. Both pure and Eu 3? -doped NiWO 4 nanostructures showed an excellent absorption in the UV region around 250 to 400 nm with the shoulder peak occurred at 450 nm. The absorption band appeared at 450 nm is mainly due to transition of electron from 3 A 2g to 1 E g [20]. The occurrence of absorption in the UV region is mainly due to excitation from O 2p to Wt 2g in the (WO 4 2-) group and the peak appeared in the visible region is due to charge transfer of oxygen 2p atom to one of the empty 5d orbital of tungsten. The broad absorption occurred at the region from 250-400 nm is due to small crystal size which in turn causes a strong quantum

Raman spectroscopy
The Raman spectra of Eu-doped NiWO 4 nanostructures are shown in Fig. 6. For monoclinic wolframite structure belonging to P 2/c space group, the Raman active vibration modes are grouped into six internal stretching modes caused by each of the six W-O bonds in the WO 6 octahedrons. Group theory calculation of the monoclinic NiWO 4 predicts 36 possible lattice modes represented as The (g) vibrations are Raman active modes and (u) modes are infrared active. For pure and Eu 3?doped NiWO 4 nanostructures show strong vibrations at 891, 778 and 698 cm -1 and weak vibrations at 552, 340, 141, 203 and 170 cm -1 are observed. It is reported that these peaks are attributed to normal W-O vibration of the WO 6 octahedra [22]. The higher intensity modes at 891 cm -1 corresponds to symmetric vibrations of short W-O bonds associated with the WO 6 symmetric stretching vibration of highly covalent bonds. The modes at 778 and 698 cm -1 are assigned to asymmetric vibrations of short W-O terminal bonds W-O-W bridges [23]. The modes observed in the frequency range between 500 and 300 cm -1 correspond to shorter and longer in-plane W-O bonds whereas the active modes below 350 cm -1 are assigned to out-of-plane vibrations of W-O bonds and lattice vibrations in the wolframite structure [24]. As the Raman active modes are quite sensitive to the nature of bonding and cation distribution at respective sites, the doping of Eu 3? at Ni 2? sites have shown a decrease in the peak intensities and considerable shift of active modes at lower frequencies. The randomize distribution of Eu substitution in NiWO 4 and changes in Eu-oxygen bond length might be responsible for these changes in the active modes.   It is clearly observed from the data that the transmittance at the focus decreases with increase in Eu 3? concentration and thus enhancing the valley. Different possible mechanisms such as two photon absorption (2PA), free carrier absorption and excited state absorption (ESA) are accountable for the observed nonlinear absorption (NLA) behavior. As the continuous laser is used in our experiment therefore the observed nonlinearity is due to thermal effects which increase ESA [25]. Thus the nonlinear mechanism leading to the observed nonlinearity is due to ESA assisted RSA in the nanostructures.
The absorption coefficient b of the nanoparticles under cw laser illumination are being estimated by the formalism.
I 0 is the intensity of the laser beam at the focus (Z = 0), L eff is the effective thickness of the sample a is the linear absorption coefficient at the laser excitation wavelength and L is the thickness of the sample. The nonlinear absorption coefficient (b) is found to be 10 -6 (cm/W). The imaginary part of the third-order nonlinear susceptibility can be determined from the nonlinear absorption coefficient by the following equation.
where c is the velocity of the light and n 0 is the linear refractive index. The closed aperture Z-scan data for the nanostructures are shown in Fig. 8. Both sign and  magnitude of the nonlinear refractive index can be determined by this method. the recorded closed aperture curves show perfocal peak followed by post focal valley indicating that the materials exhibit negative nonlinear refractive index. This signifies the self-defocusing nature of the materials. The physical origin of nonlinear refraction at cw laser regime can be electronic, molecular, electrostrictive or thermal in nature. As the cw laser produce weak electronic effect, the origin of nonlinearity cannot be explained by multiphoton absorption, therefore the originated nonlinearity is purely due to thermal effect. Due to thermal stress the thermal lens diverges and leads to variation of refractive index as a function of temperature which exhibits negative nonlinearity in the nanostructures. The material exhibiting thermal nonlinearity will focus (or) defocus the incident light. Therefore the existence of nonlinear refractive index is dominated by thermal effects.
The measurable quantity DT P-V can be obtained from the difference between the normalized peak valley and transmittance valley (TP-TV). The variation of this quantity in terms of the on-axis phase shift |u 0 | at the focus is given by [26] DT PÀV ¼ 0: where Du 0 is the on-axis phase shift at the focus, S is the aperture linear transmittance and is given by S ¼ 1 À exp À2r 2 a =x 2 a À Á is the linear aperture transmittance. r a denotes the aperture radius and x a denotes the radius of the laser spot before the aperture.
Then nonlinear refractive index is given by where k is the laser wavelength, I o = 1.1 kW/cm 2 is the input intensity and L eff is the effective length of the sample. The real part of the third-order nonlinear susceptibility v 3 ð Þ were deduced from the following equation where e o is permittivity of the vacuum, n o is the linear refractive index of the sample, and C is the velocity of light in vacuum. Finally, the absolute value of the third-order susceptibility v 3 ð Þ can be obtained given by following equation [27]: The acquired Z-scan data clearly indicates that Eu 3? doped nanostructures exhibits good third-order NLO properties compared to pure NiWO 4 which is enumerated in Table 2. The nonlinear values computed are in good agreement with the previous reported results. Tamgadge et al. reported NLO studies on Sr-CuO PVA nanocomposite thin films [28,29], where b was calculated in the order of 10 -6 cm/W. The nonlinear optical properties of Eu 3? doped NiWO 4 are higher as compared with the pure NiWO4 nanostructures which may be due to the dominant energy transfer process created by Eu ions. At higher concentration of Eu, the formation of clusters occur which decreases the Eu 3? -Eu 3? distance, leading to an effective energy transfer between the neighboring ions. The introduction of the Eu 3? dopant led to the generation of defects on the surface of NiWO 4 . Eu 3? ions on the surface of NiWO 4 could efficiently trap electrons at the conduction band, thus limiting the recombination of photogenerated charge carriers. The thermal effect also contributes may also have contribution for the observed nonlinearity by the use of cw laser beam. The thermal effect leads to thermalizing of hot electrons and subsequent dissipation of their energy leads to increase in the surrounding temperature, results in variation of refractive index change. The substitution of Eu at the lattice sites of Ni has greater influence of widening the band gaps. After the substitution of Eu 3? , the NLO appears a step rise, indicating that the possibility of enhancing the NLO effect by tuning the band gap. The high magnitude of the observed nonlinearity can be primarily due to thermal change in the sample.

Conclusions
Eu 3? -doped NiWO 4 nanostructures with various concentration of Eu 3? were prepared by a chemical precipitation technique. The structural, optical and morphological characterization as a function of doping concentration was studied. XRD results revealed that the nanostructures exhibit monoclinic wolframite structure with space group P 2/c . The variation of peak width and intensity of Raman active modes confirms the incorporation of Eu in NiWO 4 . Thirdorder nonlinear studies were done by a Z-scan technique at 532 nm using a continuous wave Nd:YAG laser. Open aperture and close aperture traces have shown that the occurrence of the observed nonlinearity is mainly due to reverse saturable absorption and negative nonlinear refractive index. The third-order nonlinear parameters were found to be increasing with increase in Eu 3? dopant concentration. The results suggest the possibility that the incorporation of Eu into NiWO 4 have potential to be used photonic device applications.