Synthesis and characterization of nickel doped zinc selenide nanospheres for nonlinear optical applications

Journal of Alloys and Compounds 791 (2019) 601e612

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Synthesis and characterization of nickel doped zinc selenide nanospheres for nonlinear optical applications R. Divya, N. Manikandan, G. Vinitha* Division of Physics, School of Advanced Sciences, Vellore Institute of Technology, Chennai, 600127, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2018 Received in revised form 19 March 2019 Accepted 21 March 2019 Available online 25 March 2019

Phase purity plays an important role in determining the properties of the material and its subsequent applications which depends on the synthesis conditions and dopant concentrations. Phase pure zinc selenide and nickel doped zinc selenide nanoparticles were synthesized by hydrothermal method using sodium selenite as precursor. Calculated crystallite size for various compositions showed a decrease with dopant concentration with the values around 10 nm and less. Morphological analysis showed the formation of spherical particles which is distinct in the micron scale image. SAED pattern showed the formation of nanocrystalline particles with particles size varying between 10 and 20 nm. Quantum confinement effect due to reduced size was corroborated with increased band gap values as measured from diffuse reflectance spectra. Photoluminescence spectra showed the presence of dominant near band-edge emission along with certain defect level emissions for all the samples. Nonlinear optical studies carried out using a continuous wave laser showed an increase in nonlinear refractive index and susceptibility values with dopant concentration signifying the role of defects and the structural modifications occurring due to doping. The nonlinear optical properties exhibited by this material has been utilized for device applications like optical limiting and the results show enhancement in limiting efficiency with dopant concentrations. © 2019 Elsevier B.V. All rights reserved.

Keywords: Zinc selenide nanospheres Hydrothermal method Nonlinear optical properties Z-scan

1. Introduction Nonlinear optics deals with various nonlinear effects which take place during the interaction between laser and matter [1]. Nonlinear optical materials which possess large third order optical nonlinearities with fast response time have become an important requisite for potential applications such as optical limiting, optical data storage, optical switching etc. The third order nonlinear optical parameters can be obtained by several techniques such as nonlinear interferometry, degenerate four wave mixing, ellipse rotation, beam distortion, Z-scan technique etc. Among all these techniques, Z-scan measurement is a simple and effective method which works on the principle of spatial distortion of Gaussian laser beam arising from nonlinear self-phase modulation (SPM) as the laser beam is passed through the material. The most important aspect of Z-scan method is that sign of nonlinear refraction and its magnitude can be easily determined [2]. The nonlinear optical properties exhibited by the materials can be exploited for various

* Corresponding author. E-mail address: [email protected] (G. Vinitha). https://doi.org/10.1016/j.jallcom.2019.03.294 0925-8388/© 2019 Elsevier B.V. All rights reserved.

optoelectronic device applications like optical limiting, optical bistability, optical switching etc. [3] One such application is optical limiting (OL) which is an effect in which power, irradiance, energy or fluence transmitted by an optical system is maintained below a maximum value called the limiting threshold for any input value. The OL effect is widely used in protecting optical sensors and components from laser damage. Below the threshold value, the optical limiter exhibits linear transmission and above this value, the output intensity remains constant. The mechanisms involved in optical limiting include nonlinear refraction and nonlinear absorption [4]. Exploring advanced and highly efficient materials for nonlinear optical applications has become a more fascinating area of research. Nanomaterials which exhibit notable nonlinear optical properties promote the design and fabrication of optoelectronic and photonic devices. Materials like Beryllium-free nitrate (Ba2NO3(OH)3) [5], Rb3VO(O2)2CO3 [6], Cs3VO(O2)2CO3 [7], Zn1-xMgxSe [8], have been studied for their nonlinear properties. In particular, various nanostructured materials like MgFe2O4 [9], Gold decorated Graphene nanocomposites [10], Cu:Al2O3 nanocomposite [11], CdSe quantum dots [12], ZnS [13], b-BaB2O4 [14], Zn1-xAgxSe crystals [15], Zn1-

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xMgxSe and Cd1-xMgxSe crystals [16] have been studied for their excellent third order nonlinear behaviour. Among various nanomaterials, family of semiconducting materials has generated specific interest for applications in the domain of optoelectronics due to their ability to tune their bandgaps with incorporation of dopants leading to variation in properties. Highly polarisable metal chalcogenide semiconductors were found to be promising materials that overcame the limitations of existing standard materials for exploiting many nonlinear effects [17]. Metal selenide nanoparticles have found significant applications in the fields of optical filters, sensors, optical recording materials, solar cells etc. In particular, zinc selenide, a wide direct bandgap semiconducting material exhibiting both zinc blende and wurtzite crystal structures has been investigated for various applications such as flat panel displays, light emitting diodes (LEDs), lasers, logic gates, transistors, photocatalysts etc [18,19]. In recent times, various works have been reported on pure and doped ZnSe nanostructures. Pure and Cu doped ZnSe quantum dots synthesized by microwave assisted solvothermal synthesis where mercaptoacetic acid (MAA) act as a capping agent is found to be suitable for developing nanoprobes in bioimaging applications [20]. Colloidal quantum dots of Europium doped ZnSe have been found to exhibit good color tunability from blue to yellow ocher, which are dictated by the concentration quenching effects. Lower europium doping led to the formation of Eu2O3 & EuO on the surface of ZnSe QD while those with higher concentrations led to yield more oxygen-rich Eu3O4 on the surface of ZnSe QD [21]. It has been shown that ZnSe nanoparticles embedded in porous Nddoped carbon nanocubes led to the formation of nanocomposites, which were highly porous. These exhibited excellent electrochemical properties and their synthesis dependent porosity in the structure led to easy Liþ ion diffusion paving the way for application in lithium ion batteries [22]. On a similar note, ZnSe materials have been explored for sodium ion batteries. ZnSe embedded in the porous carbon matrix have been shown to exhibit excellent sodium storage performance in terms of their rate performance, capacity and cyclability characteristics [23]. ZnSe-CdS nano-heterojunction grown by atomic layer deposition was found to show improved optoelectronic properties with their photovoltaic power conversion efficiency of 0.96% [24]. Wang et al. showed that the catalyst assisted growth of long ultrathin zinc selenide nanowires exhibited strong quantum confinement effect due to the size control achieved by ligands and the colloidal synthesis method adopted. Though these samples showed good optical properties, their temporal photoresponsive behaviour was shown to be poor indicating the need for modifying their synthesis conditions [25]. ZnSe nanoparticles have been synthesized by various methods like solvothermal [26], microemulsion-mediated [27], microwave [28], ball milling [29], hydrothermal [30], plasma-arc discharge method [31] etc. ZnSe nanoparticles synthesized by simple thermal treatment procedure wherein the sample was calcined at different temperatures under nitrogen atmosphere, showed an improvement in their crystalline behaviour [32]. Hydrothermal method is found to be very effective in controlling the particle size. ZnSereduced graphene oxide nanocomposite synthesized by one step hydrothermal route, exhibited excellent electrochemical performance when used in lithium and sodium ion batteries [33]. The choice of dopants like transition metals in ZnSe host matrix to improve the properties depends on various parameters like valence state, ionic radius of host and dopant metals etc [34]. Nickel was chosen for doping in ZnSe host matrix due to its smaller ionic radius among transition elements which can lead to formation of defects and can act as a suitable system to understand these defect related properties [35]. Nickel was also found to influence strongly the optical properties when incorporated in II-IV compounds. This

dopant has the ability to form deep levels inside the bandgap of the host material thereby acting as electron traps and luminescence centres [36]. It is observed from literature that the source of selenium as precursor plays a role in the phase purity of the samples. It has been reported that doping of nickel ions beyond 10 mol% led to the formation of secondary impure phase along with traces of selenium [34]. Even though ZnSe nanoparticles have been investigated for variety of applications, third order nonlinear optical properties and limiting applications of doped ZnSe nanoparticles have not been explored till date. This paper deals with the synthesis and characterization of phase pure nickel doped zinc selenide nanoparticles with sodium selenite as the precursor with an extension in doping upto 20 mol% of nickel. The structure, morphology and optical properties of the synthesized samples were investigated by various experimental techniques. Z-scan study was carried out to calculate the nonlinear optical parameters and to find the suitability of the synthesized samples for optoelectronic device applications as optical limiters. 2. Experimental 2.1. Materials Sodium hydroxide pellets, Zinc Chloride (anhydrous), Sodium Selenite, Nickel Chloride (hexahydrate) and Hydrazine hydrate (80%) of analytical grade with 99% purity were purchased from Merck, India. Without any further purification, the chemicals were used as such and deionised water was used throughout the synthesis process. 2.2. Synthesis of Zn1-xNixSe (0  x  0.2) nanoparticles Initially, 40 ml of deionised water was used to dissolve 1.6 g of sodium hydroxide pellets and was subjected to stirring until a clear solution was obtained. Zinc chloride and sodium selenite in 1:1 ratio was added to this and stirred continuously. 10 ml of Hydrazine hydrate (80 wt %), which acts as a reducing agent was added to the above mixture. Finally, nickel chloride was added and stirred until a homogenous solution was obtained. The entire solution was transferred into a teflon beaker and 20 ml of deionised water was added to it to make the volume ratio. This teflon beaker was kept inside a 100 ml autoclave, sealed and kept in hot air oven at 180  C for 4 h. Finally after cooling, the remnant liquid was taken out from the teflon beaker and the green precipitate was washed several times with deionised water and dried at 90  C to yield the final product for further analysis. 2.3. Characterization techniques The structural analysis for Zn1-xNixSe (0  x  0.2) nanoparticles was carried out using Rigaku SmartLab Automated Multipurpose Xray Diffractometer with Cu-Ka1 (l ¼ 1.541 Å) radiation as the source and 2q values ranging from 10 to 80 . The functional groups of the synthesized materials were obtained from Perkin Elmer FTIR instrument in the scan range of 400e4000 cm1. UVeVisible Diffuse reflectance spectra (DRS) were recorded in the spectral range of 200e800 nm by means of Thermo scientific evolution 300 UVeVisible spectrophotometer. Photoluminescence spectra were obtained using Perkin Elmer, LS45, fluorescence spectrometer. The surface morphology and the elemental composition of the synthesized particles were obtained from Joel JSM 6360 high resolution scanning electron microscope (HR-SEM) along with energy dispersive x-ray (EDX) analyzer. Jeol/JEM 2100 high resolution transmission electron microscope was used to obtain HR-TEM

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images. The third order nonlinear parameters were obtained from CW 532 nm diode laser. Optical limiting studies have been carried out using Q-switched Nd: YAG laser of 9 ns pulse width (New port USA (2008)). 2.4. Reaction mechanism of the synthesized samples 2.4.1. Choice of selenium source Based on literature reports, commonly used selenium sources are selenium (Se) metals basis and sodium selenite (Na2SeO3). In this work, sodium selenite was chosen as selenium source, since metal selenium doesn't dissolve in water easily leading to incomplete reaction and formation of selenium residue in the final products which cannot be eliminated easily [37]. 2.4.2. Role of hydrazine hydrate and sodium hydroxide In this work, hydrazine monohydrate (N2H4$H2O) acts as a strong reducing agent, thereby regulating the morphology of the samples to be spherical and also controlling the phase formation of the final product. ZnSe microstructures can be obtained by using appropriate amounts of hydrazine monohydrate and sodium hydroxide [37]. In the present work, ZnCl2 and Na2SeO3 were used as precursor materials. SeO2 3 ions were reduced to selenium atoms by the addition of hydrazine hydrate. Zn2þ ions which were released from ZnCl2 react with Se2 ions forming zinc selenide (ZnSe) monomers. The nanocrystallites tend to form as aggregates and N2 gas bubbles which were produced during the reaction act as aggregation centres. Since the interfacial energy needs to attain a minimum, the formed ZnSe nanocrystallites prefer to aggregate in the gas-liquid interface between N2 and water. N2 gas bubbles act as a soft template resulting in the formation of ZnSe nanospheres [38]. 3. Results and discussion 3.1. Powder X-ray diffraction analysis

where bD/ peak width at half maximum intensity.

Debye-Scherrer relation was used to find out the average crystallite size (L) for the synthesized Zn1-xNixSe (0  x  0.2) nanoparticles by considering the (111) diffraction peak.

L¼

The phase formation and crystal structure was found using powder x-ray diffraction study. XRD patterns of Zn1-xNixSe (0  x  0.2) nanoparticles are depicted in Fig. 1. The XRD patterns match well with JCPDS data (card no. 37-1463). The diffraction peaks of the samples were assigned to (111), (200), (311), (400) and (331) planes of nickel doped zinc selenide which exhibits cubic zinc blende structure. From the XRD spectra, it is clearly observed that diffraction peaks of doped samples are slightly shifted towards higher angles compared to pure zinc selenide as the concentration of nickel ions is increased. The maximum shift in the peak between   pure ZnSe (2q ¼ 27.252 ) and Zn0.8Ni0.2Se (2q ¼ 27.480 ) is found to  be Dq ¼ 0.228 . The primary reason for shifting of peaks to higher angles is attributed to the divalent Ni2þ ions occupying sites of Zn2þ ions [34]. The purity of the synthesized sample is confirmed by the absence of any additional or impure peaks. Better crystalline nature of the samples is confirmed by the formation of narrow intense peaks. The combination of instrument and sample dependent effects gives the width of the Bragg peak. The diffraction pattern from the line broadening of standard material e.g. silicon is obtained in order to decouple all the contributions [39]. The instrument-corrected broadening with respect to the diffraction peak of ZnSe was evaluated using the relation

b2D ¼ ½b2ðmeasuredÞ  b2ðinstrumentalÞ 

Fig. 1. Powder X-ray diffraction pattern of Zn1-xNixSe (0  x  0.2) nanoparticles.

(1)

0:89l b cos q

(2)

where L is the crystallite size, l, the wavelength of the x-ray source (1.541 Å), b, the full width at half maximum (FWHM) of the diffraction peak and q, the Bragg angle. The calculated values lie in the range of 8e12 nm with their sizes decreasing with increase in nickel concentration. Bragg's relation as given below was used to calculate the interplanar spacing (d),

d¼

nl 2 sin q

(3)

Since the synthesized Zn1-xNixSe (0  x  0.2) nanoparticles exhibit cubic structure, the lattice constant ‘a’ (a ¼ b ¼ c) was determined from

a ¼ dðh2 þ k2 þ l2 Þ0:5

(4)

Calculated lattice parameters were found to follow Vegard's law indicating a decrease in lattice constant value with systematic doping of smaller sized nickel ions in the zinc selenide matrix [34]. The reduction in lattice constant and the crystallite sizes with increase in nickel concentration indicates that nickel favours the size reduction, overcoming the barrier for reduction exhibited by host zinc selenide matrix under similar processing conditions. The number of defects present in the sample defined as dislocation density is given by the relation:

604

d¼

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1 D2

(5)

where, D is the crystallite size and d is the dislocation density corresponding to the crystallographic defects present in the lattice [40]. In Zn1-xNixSe (0  x  0.2) nanoparticles, the dislocation density value increases as the concentration of the dopant material increases and the values are given in Table 1. The strain and effective crystallite size of all the synthesized samples were obtained from the Williamson-Hall plot by considering FWHM of their corresponding diffraction peaks. Inclusion of nickel as dopant in zinc selenide matrix gives rise to strain in the matrix. FWHM can be obtained from the linear combination of strain (ε) and effective crystallite size (L) using the following relation [41].

bcosq k 4εsinq ¼ þ D l l

(6)

Williamson-Hall plot of b Cosq/l vs 4Sinq/l is depicted in Fig. 2 which is found to be linear. The effective crystallite size is obtained from the reciprocal of intercept on b Cosq/l axis whereas the strain can be obtained from the slope of the plot. The effective crystallite size decreases whereas the strain value increases when the concentration of nickel dopant increases. The positive slope obtained from WeH plot indicates the presence of tensile strain in the synthesized samples [42].

Fig. 2. W-H plot of Zn1-xNixSe (0  x  0.2) nanoparticles.

3.2. Spectroscopic studies 3.2.1. Fourier transform infrared spectroscopy (FTIR) The mid IR (400 cm1- 4000 cm1) spectra of pure and nickel doped zinc selenide samples are depicted in Fig. 3. The functional groups and their corresponding wavenumbers are tabulated in Table 2. The peaks which appear around the region 490 cm1 is assigned to the ZneSe stretching mode. The bending vibrational mode of ZneSe appears at 740 cm1 [43]. Vibrations corresponding to hydroxyl groups occur at 3430 cm1 indicating the presence of small traces of water molecules in the sample [44]. 1354 cm1 band corresponds to CeH stretching vibrations. The bands in the region at 1217 cm1 correspond to CeO stretching, while that at 1706 cm1 confirms the presence of C]O stretching [45]. The weak band around 2921 cm1 indicates an asymmetric stretching mode vibration of CH2 group and the band around 1041 cm1 correspond to C]O stretching vibration modes. The doping of nickel ions in the host lattice (ZnSe) is confirmed by the slight shift of bands in the spectra [34]. 3.2.2. UV- diffuse reflectance spectroscopy (UV-DRS) The synthesized Zn1-xNixSe (0  x  0.2) nanoparticles were characterized using UV-DRS to obtain the bandgap of the synthesized nanoparticles. Reflectance data was recorded in the 200 nm800nm region. The bandgap of the synthesized samples was obtained from Tauc plot (Fig. 4) using reflectance data. Using Kubelka-

Fig. 3. FTIR spectrum of Zn1-xNixSe (0  x  0.2) nanoparticles.

Munk function, the obtained reflectance data was utilized and the optical absorption coefficient was obtained from the relation

a ¼ FðRÞ ¼

ð1  RÞ2 2R

(7)

where R is the reflectance, a is the absorption coefficient and F(R) is

Table 1 Calculated values of crystallite size, effective crystallite size, strain, lattice parameter, interplanar spacing and dislocation density of Zn (1-x) Nix Se (0  x  0.2) nanoparticles. Sample Code Crystallite size (nm)

Effective crystallite size (nm)

Interplanar spacing (dhkl) in Å Lattice parameter, (a) in Å Strain (Ɛ)

Dislocation density, (d) *1015 line/m2

ZnSe Zn0.95Ni0.05Se Zn0.9Ni0.1Se Zn0.85Ni0.15Se Zn0.8Ni0.2Se

11 10 9 8 7

3.270 3.267 3.264 3.257 3.251

6.94 8.26 10 12.34 15.62

12 11 10 9 8

5.663 5.658 5.653 5.641 5.630

0.1050 0.1380 0.1469 0.2210 0.2513

R. Divya et al. / Journal of Alloys and Compounds 791 (2019) 601e612 Table 2 Functional groups and their corresponding wave numbers. S.No

Wavenumber (cm1)

Assignment

1. 2. 3. 4. 5. 6. 7. 8.

490 740 3430 1354 1217 1706 2921 1041

ZneSe stretching mode ZneSe bending vibrational mode Presence of hydroxyl groups CeH stretching vibrations CeO stretching C¼O stretching CH2 group asymmetric stretching mode C¼O stretching vibration modes

the Kubelka-Munk function. The band gaps of the synthesized samples were obtained using Tauc relation

FðRÞ hy ¼ Aðhy  Eg Þn

(8)

where hn is the photon energy in eV, Eg is the bandgap energy, A is a constant and n takes the value 0.5 for indirect transition and 2 in case of direct transition [46]. The square of Kubelka-Munk function, [F(R)hn]2 vs energy (eV) is plotted and the linear part of the curve is extrapolated by

605

considering [F(R)2] ¼ 0 [47]. The values of bandgap energy for Zn1xNixSe (0  x  0.2) nanoparticles were found to be 2.39 eV, 2.47 eV, 2.55 eV, 2.76 eV and 2.80 eV for x ¼ 0, 0.05, 0.10, 0.15 and 0.20 respectively. It is evident from the calculated band gap values that as the dopant concentration (Ni2þ ions) increases, there is an increase in the value of band gap which indicates a blue shift in Zn1xNixSe (0  x  0.2) samples when compared to their bulk counterparts. Considering the obtained smaller values of effective crystallite sizes, this blue shift could be attributed to the quantum confinement effect of an electron-hole pair in the synthesized materials [48]. 3.2.3. Photoluminescence spectroscopy Photoluminescence (PL) spectra were recorded at room temperature in order to examine the electronic transitions which take place between valence and conduction bands [49]. PL spectra of Zn1-xNixSe (0  x  0.2) nanoparticles recorded at room temperature is depicted in Fig. 5a and the near band edge emission of Zn1xNixSe (0  x  0.2) nanoparticles is shown in Fig. 5b. The excitation wavelength was fixed at 254 nm. A sharp, high intense green emission peak at 517 nm (2.39eV) indicating the near band edge (NBE) emission is observed which is in accordance with the bandgap value obtained from UV-DRS spectra.

Fig. 4. Plot of (ahy)2 vs hy of Zn1-xNixSe (0  x  0.2) nanoparticles.

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Fig. 5. a) Photoluminescence Spectrum of Zn1-xNixSe (0  x  0.2) nanoparticles; b) Near band edge emission spectra of Zn1-xNixSe (0  x  0.2) nanoparticles.

In general, emission peaks relatively at higher wavelengths are associated with electron-hole recombination mainly from zinc interstitial defect centres in addition to the photo-generated holes from valence band [45]. From PL spectra, it was observed that there is no change in the peak position as the dopant concentration increases. Further, it was noted that the emission peak intensity increases, as the concentration of doping increases corresponding to excitons (e- hole pairs) which are trapped in the shallow region [48]. The emission peak at 567 nm (2.18eV) is a defect level emission. The weak green emission band corresponds to the self activated luminescence due to some donor-acceptor pairs of ZneSe vacancy and interstitial states [50]. The weak infrared emission at 776 nm (1.59eV) is mainly due to selenium vacancies which act as deep defect level donors thereby leading to the formation of new energy levels resulting in rare infrared emission [51]. The strength of emission intensity depends on concentration of the dopant (nickel). In the present case, it was found that as the dopant concentration increases, there is an increase in the emission intensity [52]. 3.3. Morphological analysis 3.3.1. Scanning electron microscopy (SEM) HR-SEM images of pure ZnSe is shown in Fig. 6a and b and representative nickel doped Zn0.9Ni0.1Se samples are shown in Fig. 6c and d, which reveal the surface morphology. It is evident from the image that the particles are formed in spherical shape with good monodispersity. The mechanism behind the formation of ZnSe nanospheres is attributed to Ostwald ripening. ZnSe monomers formed initially due to the reaction between the constituent chemicals acted as nucleation centres leading to the growth of ZnSe nanocrystals. This growth is driven by the principle of total energy minimization in the system allowing ZnSe nanocrystals to combine and form as solid nanospheres. Ostwald ripening mechanism involves the growth of smaller sized crystals into larger crystals due to the higher solubility of smaller crystals than larger crystals [53]. The particle size of pure ZnSe nanoparticles is in the range of 22e45 nm and for nickel doped ZnSe, the particle size is in the range around 18e35 nm. 3.3.2. Energy dispersive x-ray (EDAX) analysis EDAX analysis gives comprehensive information about the chemical composition of ZnSe and representative Zn0.9Ni0.1Se nanoparticles as depicted in Fig. 7a and b. The presence of individual zinc, selenium and nickel atoms in the synthesized samples

is confirmed from the observed spectra which also show the fraction (in atomic and weight %) of the elemental composition. The stoichiometric composition of the synthesized samples matches well with the atomic ratio of experimental quantity. There is an additional peak at 2.1 keV corresponding to gold which was coated on the surface of the samples by sputtering. 3.3.3. High resolution transmission electron microscopy (HR-TEM) HR-TEM images of pure ZnSe and representative Zn0.9Ni0.1Se nanoparticles are depicted in Fig. 8a and b. The nanoparticles are found to be homogenous, agglomerated and spherical in nature. The high surface energy of the nanoparticles leads to the aggregation of particles [54]. The effect of dopant in the host material could be clearly observed from the particle size data which indicates that the size distribution from 10 to 35 nm for pure sample got reduced to 10e20 nm for samples doped with 10 mol% of nickel. This size reduction indicates that the doped samples behave like Quantum dots. The interplanar ‘d’ spacing in Fig. 8c and d obtained from HRTEM images for pure and Zn0.9Ni0.1Se nanoparticles is found to be 0.35 nm and 0.30 nm respectively which corresponds to the interplanar distance of (111) plane of cubic ZnSe. The obtained ‘d’ spacing values from HR-TEM are found to be consistent with ‘d’ spacing values obtained from XRD data. The selective area diffraction (SAED) pattern for pure ZnSe and Zn0.9Ni0.1Se nanoparticles is depicted in Fig. 8e and f respectively. A series of diffused rings with number of bright spots arranged orderly in a circular pattern indicates the nature of the synthesized material to be polycrystalline. The bright spots arise due to the random orientation of the nanomaterials which correspond to the diffraction pattern obtained from various atomic planes of the nanomaterials [34]. 3.4. Nonlinear optical studies 3.4.1. Z -scan studies Z-Scan is a flexible technique in which sample is translated through the focal plane of Gaussian laser beam leading to changes in the transmittance value in the far field position. In case of refractive nonlinearity, intensity dependent nonlinear phase is induced by the light field and as a result of this, the sample acts as a lens which is due to the transverse Gaussian intensity profile. The incident beam is either defocused or re-collimated by the induced nonlinear phase which is primarily dependent on the z position with respect to the focal plane. The sample is moved along the

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Fig. 6. HR-SEM images of a) Pure ZnSe (5 mm) b) Pure ZnSe (400 nm). c) Zn0.9Ni0.1 Se (5 mm) d) Zn0.9 Ni0.1 Se (400 nm) nanoparticles.

Fig. 7. EDX spectrum of a) Pure ZnSe b) Zn0.9 Ni0.1 Se nanoparticles.

direction of propagation of laser beam, back and forth the focal point of the lens. The change in transmittance value as measured through a finite circular aperture placed just before the photodetector in the far field gives the closed aperture Z-scan

signature. In case of open aperture measurement, the aperture is replaced by a lens, which is used to collect the entire transmitted beam. The thickness of the sample is considerably small when compared to the diffraction length of the laser beam and hence the

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Fig. 8. HR-TEM images of a) Pure ZnSe and b) Zn0.9 Ni0.1 Se nanoparticles; Interplanar spacing of c) Pure ZnSe and d) Zn0.9 Ni0.1 Se nanoparticles; SAED pattern of e) Pure ZnSe and f) Zn0.9 Ni0.1 Se nanoparticles.

thin sample approximation holds good. The sample is moved along the direction of propagation of laser beam through the lens and the transmittance is measured through the photodetector which gives the closed aperture Z-scan signature. Z-scan technique helps in finding nonlinear parameters like nonlinear refractive index, nonlinear absorption coefficient and third order nonlinear susceptibility. The consequence of doping nickel in ZnSe matrix has been investigated by using CW laser excitation (532 nm, 100 mW). All the synthesized samples were dispersed in diethylene glycol (at 63% transmittance) and taken in a 1 mm cuvette. Rayleigh length (ZR) was found to be 1.52 mm which satisfies the condition ZR > L, where L is the path length. The calculated beam waist (ɷ0) was found to be 16.06 mm. Z-Scan parameters are tabulated in Table 3. The cuvette was placed on the translation stage and translated along the axial path of the propagation of the laser beam. Closed, open and closed to open ratio Z-

scan curves of Zn1-xNixSe (0  x  0.2) nanoparticles are shown in Fig. 9(aec). The peak followed by valley-normalized transmittance for Zn1-xNixSe (0  x  0.2) samples is depicted in Fig. 9a which reveals that the nonlinear refraction is negative i.e. self-defocusing, that arises due to the local variation in the refractive index with temperature [14]. Open aperture Z-scan as in Fig. 9b reveals reverse saturable absorption (RSA) as evident from the valley. Since closed aperture gives both nonlinear refraction as well as absorption and open gives pure nonlinear absorption, ratio of closed to open is taken in order to get pure nonlinear refraction (Fig. 9c). When CW laser passes through the sample, the localised rise in temperature gives rise to thermal nonlinearity. As the laser is continuously incident on the material, a temperature difference is created and leads to thermal lensing, thermal birefringence and fracture. The spatial variation in refractive index arises due to nonuniform temperature distribution. The optical nonlinearity is

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Table 3 Z Scan experimental parameters. Z-Scan experimental parameters 1 2 3 4 5 6

Wavelength of the laser Focal length of the lens Aperture radius, (ra) Path length (L) Effective thickness (Leff) Beam waist at the aperture, (ua)

532 nm 103 mm 1.25 mm 1 mm 0.99 mm 2.5 mm

Fig. 9. a) Closed aperture; b) Open aperture; c) Closed to Open ration z-scan curves of Zn1-xNixSe (0  x  0.2) nanoparticles.

classified as instantaneous nonlinearity which is dependent on the instantaneous intensity inside the medium and accumulative nonlinearity that is dependent on energy density deposited in the medium. The nonlinear refraction (n2) obtained from closed aperture Z-scan data is used to calculate the real part of susceptibility (Re(c3)). Nonlinear absorption coefficient (b) is used to calculate the imaginary part of susceptibility (Im(c3)). The standard equations were used to calculate n2, b, and c3 [55]. Nonlinear refractive index and the third order nonlinear susceptibility (c3) values tend to increase as the dopant (Ni2þ ions) concentration increases. The systematic increase in these parameters with dopant concentration shows the effect of nickel ions in zinc selenide matrix. It can be attributed to the more polarisable nature of nickel ions. The presence of defect states is observed in the lattice as nickel ions are introduced in ZnSe matrix. By exciting a photon of certain energy the carriers may get excited to the defect level, as a result of which free carrier absorption takes place thereby leading to a rise in nonlinearity in the nanoparticles. Hence from

the results it is clearly evident that nonlinear behaviour is highly associated with the structural property changes in the material [56]. The calculated third order nonlinear parameters such as n2, b and c3 are tabulated in Table 4. The obtained nonlinear values indicate that these materials can be exploited for device applications like optical limiting.

3.4.2. Optical limiting studies Optical limiting studies were performed with slight modification in Z-scan setup in order to study the fluence-dependent light transmission measurements using Q-switched Nd: YAG laser of wavelength 532 nm with pulse width 9 ns. At higher input fluence, an ideal optical limiter becomes opaque and completely transmits at low input fluence. The optical limiting measurement for Zn1xNixSe (0  x  0.2) nanoparticles was carried out by dispersing the samples in diethylene glycol. The linear transmittance for all the samples is around 65%. Input power was varied and corresponding output was recorded. It is obvious that the lower the optical

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Table 4 Third order nonlinear parameters of Zn

(1-x)

Nix Se (0  x  0.2) nanoparticles with an error of ±3% in n2 and ±0.5% in b.

Sample Code

Linear refractive index, (n0)

Linear absorption coefficient (@ 532 nm)

n2 x 108 cm2/W

b x 103 cm/W

c3 x 106 esu

ZnSe Zn0.95Ni0.05Se Zn0.9Ni0.1Se Zn0.85Ni0.15Se Zn0.8Ni0.2Se

1.73 1.86 1.79 1.74 1.63

1.058 1.078 1.133 1.249 1.436

1.48 1.64 2.83 3.10 3.11

1.96 2.05 1.80 1.78 1.69

1.28 1.62 2.37 2.48 2.52

limiting threshold better is the optical limiting performance. It is evident from Fig. 10 that with increase in dopant concentration the limiting threshold decreases which confirms the increase in limiting efficiency. The optical limiting thresholds were found as 4.45, 6.69, 3.46, 3.35 and 2.32 (1012 W/m2) for nickel concentrations of 0, 0.05, 0.1, 0.15 and 0.2 respectively. The obtained results reveal that Zn1-xNixSe (0  x  0.2) nanoparticles are found to be good optical limiters [57].

4. Conclusions Phase pure undoped and nickel doped zinc selenide nanoparticles were synthesized by hydrothermal method with temperature maintained at 180  C. The samples showed better nanocrystallinity with their effective crystallite sizes lying well below 15 nm. The calculated optical bandgap values from the measured reflectance spectra indicated a blue shift which was

Fig. 10. Optical limiting curves of Zn1-xNixSe (0  x  0.2) nanoparticles.

R. Divya et al. / Journal of Alloys and Compounds 791 (2019) 601e612

correlated to the quantum confinement effect based on the size of these formed nanoparticles. Near band edge and defect level emissions were confirmed from the luminescence measurements, while the functional groups were identified from the infrared spectra. Ostwald ripening mechanism led to the formation of spherical particles and the SAED pattern confirmed their nanocrystalline nature. The prominent nonlinear optical properties showed a systematic increasing trend due to the structural transformations occurring in the zinc selenide network with doping of nickel. Observed enhancement in limiting threshold efficiency with doping paves way for utilizing these materials in optoelectronic devices like limiters. Acknowledgements The authors gratefully acknowledge Department of Atomic Energy- Board of Research in Nuclear Sciences (DAE-BRNS), Government of India for their support by funding this Research work (Sanction Number: 34/14/55/2014-BRNS). References [1] Yu-xi Zhang, Yu-hua Wang, Nonlinear optical properties of metal nanoparticles: a review, RSC Adv. 7 (2017) 45129e45144. [2] S. Jeyaram, T. Geethakrishnan, Third-order nonlinear optical properties of acid green 25 dye by Z-scan method, Opt. Laser. Technol. 89 (2017) 179e185. [3] A. Suresh, N. Manikandan, G. Vinitha, N-Methyl Urea Succinic acid (NMUSA): an optically non-linear organic crystal for NLO device application, Mater. Res. Express 6 (2019), 025102. [4] Natalia A. Zulina, Mikhail A. Baranov, Kirill I. Kniazev, Viacheslav O. Kaliabin, Igor Yu. Denisyuk, Susan U. Achor, Vera E. Sitnikova, Nonlinear absorption enhancement of AuNPs based polymer nanocomposites, Opt. Laser. Technol. 103 (2018) 396e400. [5] Xuehua Dong, Ling Huang, Qingyu Liu, Hongmei Zeng, Zhien Lin, Dingguo Xu, Guohong Zou, Perfect balance harmony in Ba2NO3(OH)3: a beryllium-free nitrate as a UV nonlinear optical material, Chem. Commun. 54 (45) (2018) 5792e5795. [6] Guohong Zou, Hongil Jo, Seong-Ji Lim, Tae-Soo You, Ok Kang Min, Rb3VO(O2)2CO3: a “Four-in-One” carbonatoperoxovanadate exhibiting extremely strong second-harmonic generation response, Angew. Chem. Int. Ed. 57 (28) (2018) 8619e8622. [7] Guohong Zou, Zhien Lin, Hongmei Zeng, Hongil Jo, Seong-Ji Lim, Tae-Soo You, Ok Kang Min, Cs3VO(O2)2CO3: an exceptionally thermostable carbonatoperoxovanadate with the extremely large second harmonic generation response, Chem. Sci. 9 (2018) 8957e8961. [8] B. Derkowska, B. Sahraoui, X. Nguyen Phu, G. Glowacki, W. Bala, Study of linear optical properties and two-photons absorption in Zn1-xMgxSe thin layers, Opt. Mater. 15 (2000) 199e203. [9] M. Saravanan, T.C. Sabari Girisun, G. Vinitha, Third-order nonlinear optical properties and power limiting behaviour of magnesium ferrite under CW laser (532 nm, 50 mW) excitation, J. Mater. Sci. 51 (6) (2016) 3289e3296. [10] Prabin Pradhan, Podila Ramakrishna, Muralikrishna Molli, Adarsh Kaniyoor, V. Sai Muthukumar, S. Siva Sankara Sai, S. Ramaprabhu, A.M. Rao, Optical limiting and nonlinear optical properties of gold-decorated graphene nanocomposites, Opt. Mater. 39 (2015) 182e187. [11] R. del Coso, J. Requejo-Isidro, J. Solis, J. Gonzalo, C.N. Afonso, Third order nonlinear optical susceptibility of Cu:Al2O3 nanocomposites: from spherical nanoparticles to the percolation threshold, J. Appl. Phys. 95 (5) (2004) 2755e2762. [12] S.M. Ma, J.T. Seo, Q. Yang, R. Battle, H. Brown, K. Lee, L. Creekmore, A. Jackson, T. Skyles, B. Tabibi, S.S. Jung, W. Yu, M. Namkung, Third-order nonlinear susceptibility and hyperpolarizability of CdSe nanocrystals with femtosecond excitation, J. Korean Phys. Soc. 48 (6) (2006) 1379e1384. [13] Z. Dehghania, Z. Shadrokh, M. Nadafan, The effect of magnetic metal doping on the structural and the third-order nonlinear optical properties of ZnS nanoparticles, Optik 131 (2017) 925e931. [14] T.C. Sabari Girisun, M. Saravanan, G. Vinitha, Role of reaction time in tuning the morphology and third order nonlinear optical properties of barium borate, Opt. Laser. Technol. 89 (2017) 54e58. [15] B. Derkowska, B. Sahraoui, X Nguyen Phu, G. Glowacki, W. Bala, Experimental study of the third-order optical nonlinearities in Zn1xAgxSe crystals, J. Opt. A Pure Appl. Opt. 2 (2000) 515e518. [16] B. Derkowska, Z. Essaidi, B. Sahraoui, A. Marasek, F. Firszt, M. Kujawa, Nonlinear optical properties of Zn1-xMgxSe and Cd1-xMgxSe crystals, Opt. Mater. 31 (2009) 518e522. [17] J.I. Jang, D.J. Clark, J.A. Brant, J.A. Aitken, Y.S. Kim, Highly efficient infrared optical nonlinearity of a wide-bandgap chalcogenide Li2CdGeS4, Opt. Lett. 39 (15) (2014) 4579e4582.

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