Nano-IJNA-STUDY OF NONLINEAR OPTICAL

International Journal of Nanotechnology and Application (IJNA) ISSN(P): 2277-4777; ISSN(E): 2278-9391 Vol. 4, Issue 3, Jun 2014, 21-28 © TJPRC Pvt. Ltd.

STUDY OF NONLINEAR OPTICAL PROPERTIES OF SODIUM DOPED V2O5 NANOPARTICLES PRABIN PRADHAN, MURALIKRISHNA MOLLI, LAKSHMAN KUMAR V, LACHIT SAIKIA, V. SAI MUTHUKUMAR, S. SIVA SANKARA SAI & K. VENKATARAMANIAH Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, Anantapur, Andhra Pradesh, India

ABSTRACT We report here the nonlinear optical (NLO) properties of vanadium pentoxide (V2O5) nanoparticles and improvement in the optical limiting performance due to sodium doping in V2O5. In this study, V2O5 and Na-doped V2O5 nanoparticles were synthesized by solution combustion method. The as-synthesized samples were further characterized using XRD, FESEM, EDAX, TEM, and UV-Visible spectroscopy. The electrical conductivity measured using Van der Pauw method showed that the electrical conductivity increased with increase in doping concentration. Open-aperture z-scan technique was employed to study the NLO absorption behavior of the synthesized samples using a second harmonic (532 nm) of Nd: YAG laser with 15 ns pulse width. We observed that 5 mol % Na-doped V2O5 exhibited enhanced nonlinear absorption compared to undoped V2O5 and 3 mol % Na- doped V2O5 which we attribute to the enhanced free carrier concentration due to Na doping. The mechanism of nonlinear absorption (NLA) was found to be a three photon absorption process.

KEYWORDS: Optical Limiting, Z-Scan, V2O5, Na-Doped V2O5, Nonlinear Absorption, Nonlinear Scattering INTRODUCTION Transition metal oxides with different morphological structure find their way in various potential applications. There are series of oxides that form from vanadium which are commonly available as VO, V2O3, VO2, and V2O5. The most important of these oxides is V2O5, and is the most stable member of the series [1].Vanadium pentoxide (V2O5) has been in the forefront of applied research in view of its multifunctional properties, which include optical, electronic and electrochromic properties [2]. Nanostructures of V2O5 are currently used in lithium-ion battery, super capacitors, field emission, chemical and biosensors and for fabricating various nanodevices. Also, vanadium oxides have been employed in photonic applications like surface-enhanced Raman spectroscopy and nonlinear optical studies [3].Nonlinear optical studies were reported on some nanostructures of vanadium oxides in the form of nanotubes [4], nanoflowers [3] and thin films [5]. In this study, we report the nonlinear optical absorption properties of V2O5 nanoparticles and Na-doped V2O5 nanoparticles. We found that Na doping has enhanced the nonlinear absorption of V2O5 nanoparticles.

EXPERIMENTAL Synthesis V2O5 and Na-doped V2O5 nanoparticles were synthesized through solution combustion method. To synthesize vanadium pentoxide, 2 g of ammonium metavanadate, 1.28 g glycine and 8 g ammonium nitrate were dissolved in 15 ml of distilled water. The mixture was taken in a 300 cm3pyrex dish and introduced into a muffle furnace (38×17×9 cm3)

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Prabin Pradhan, Muralikrishna Molli, Lakshman Kumar V, Lachit Saikia, Saikia V. Sai Muthukumar,, S. Siva Sankara Sai & K. Venkataramaniah

maintained at 5000C.The mixture boils, ignites, ignites, and burns with an incandescent flame of yellow-green yellow color with an immense evolution of gaseous products in the process. The product of combustion is orthorhombic V2O5, which occupies the entire volume of the container. The entire combustion process was completed in about 3 min producing about 1.55 g of yellowish orange colored vandia.

Figure 1: a) PureV2O5 b) Na-Doped V2O5 Obtained from Solution Combustion Synthesis Na was added into the matrix of vanadium pentoxide using their corresponding metal salts. The salt used for this study was sodium nitrate. Appropriate amount of the sodium nitrate was added to the stoichiometric mixture of ammonium metavanadate, glycine and ammonium nitrate to synthesize 3 mol% and 5 mol% Na-doped Na doped V2O5. The nature of the combustion was vigorous. The Na-doped doped sample showed a significant color change. There was wa a color change from orange to dark brown (shown in figure 1).

RESULTS AND DISCUSSIONS X-Ray Diffraction Studies The powder XRD patterns for V2O5, 3 mol% Na-doped V2O5and 5 mol% Na- doped V2O5 samples were recorded using PAN alytical X'pert Pro MPD diffracto meter with the following settings: voltage 45 kV and current 30 mA. mA The XRD patterns shown in figure 2 show that all a the samples are crystalline. The powder XRD pattern of V2O5 can be indexed with orthorhombic V2O5 (JCPDS No: 01-072-0433) 01 0433) with unit cell parametersa = 11.51Å, b = 4.36 Å, c = 3.56 Å, α = β = γ = 900. The powder XRD pattern of the samples which are doped with Na can be indexed with orthorhombic V2O5 and monoclinic NaV6O15. Peaks labelled with * and # in the XRD patterns distinguish the NaNa doped samples from undoped V2O5. The crystallite size of all the samples were calculated using Debye Scherrer formula and tabulated in table 1. Silicon was used as the standard to subtract the instrument’s instrument contribution to the peak broadening. The crystallite size values show that the samples are nanocrystalline.

Impact Factor (JCC): 1.8003

Index Copernicus Value (ICV): 3.0

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Study of Nonlinear Optical Properties of Sodium Doped V2O5 Nanoparticles

Figure 2: XRD Patterns for Pure V2O5, 3mol% Na-Doped V2O5 and 5mol% Na-Doped Doped V2O5 Nanoparticles Table 1:: Crystallite Size of V2O5, 3 mol% Na-Doped V2O5 and 5 mol% Na-Dopedv Na 2 o5 Sample V 2O 5 3 mol % Na- V2O5 5 mol % Na- V2O5

Crystallite Size (nm) 163 82 163

Electron Microscopy The electron micrograph of the sample was collected using FESEM (Zeiss Model Gemini Ultra 55). The FESEM image of V2O5 is shown in figure 3. The size of the particles is around 200 nm.

Figure ure 3: FESEM Micrograph of V2O5 Nanoparticles Elemental analysis was performed using EDAX. The atomic percentage of V and O was found to be 28.57% and 71.43% respectively. No other element was found showing that the sample was not contaminated. Sodium could not be identified using EDAX as the doping percentage p was very small. The presence of Na was confirmed from XRD data as well as electrical conductivity measurements where we found that Na doping has actually increased the electrical conductivity which is reported in the electrical conductivity data. The TEM micrograph and electron diffraction pattern of V2O5 were recorded using HRTEM (Tecnai F30, FEI, Eindhoven, Netherlands and 200 kV). The TEM micrograph and electron diffraction pattern are shown in the figure fig 4.

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Figure 4: (a) Electron Diffraction Pattern and (b) TEM Micrograph of V2O5 Nanoparticles UV-Visible Absorption Spectroscopy The absorption spectra of the samples were recorded using UV-Visible UV Visible Spectrophotometer (Shimadzu UV-2450). UV Samples were dispersed in HPLC water and the spectra were recorded between 200 and 800 nm. The UV-Visible UV absorption spectra for V2O5, 3 mol% and 5 mol% Na-doped Na V2O5 samples les are shown in the figure 5.

Figure 5: UV-Vis Spectra for Pure V2O5, 3 mol% and 5 mol% Na-Doped Doped V2O5 Electrical Conductivity Measurements The electrical conductivity of pure V2O5, 3 mol% and 5 mol% Na-doped V2O5 was measured using Van der Pauw method. The plot of conductivity vs. temperature is shown in figure fig 6.



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Figure 6: Electrical Conductivity Measurements of V2O5, 3 mol% and 5 mol% Na- Doped V2O5 The plot shows increase in conductivity of Na-doped V2O5 samples which can be attributed to increase in free carrier concentration. With increase in doping concentration of Na the electrical conductivity is increased. Nonlinear Optical Studies We employed popular open aperture z-scan technique [6] to study the NLO properties of these samples. In particular, we adopted open aperture z-scan configuration to measure nonlinear optical absorption and scattering losses. In a conventional open aperture z-scan setup, the sample of interest is translated across the focal plane of a converging lens which focuses the incident laser beam. The transmittance of the sample is recorded as a function of sample position by a calibrated photodetector. The temporal fluctuations of the incident laser pulses are monitored through another reference photodetector. We have used a high power Nd: YAG laser with 532 nm excitation line and 15 ns pulse width. The repetition of laser pulses was restricted to 1 Hz and the whole experiment was automated using Lab VIEW interfacing that enabled easier sample translation and data collection. In order to infer and quantify the presence of nonlinear scattering in these samples, we fixed a third photodetector at about 450 on the same translation stage in very close proximity to the sample holder. 1.8 mg of each sample was dispersed in 5 ml HPLC water in three different 5 ml sample bottles. Each sample was taken in a 1mm thick fused silica cuvette and the linear transmittance of the samples was adjusted to be around 70% at 532nm. The samples were found to exhibit large optical nonlinearity, leading to optical limiting (OL) behavior. Optical limiting can be due to a variety of nonlinear optical processes such as reverse saturable absorption (RSA), multiphoton absorption, and thermal scattering. Optical limiters based on nonlinear absorption mechanisms are known to be very efficient[3].We find that the data fits best to a three-photon type absorption process, given by the expression [7] =

∝

∞ ln ∞

1+

exp −2

+

exp −

!"

(1)

Here, z is the sample position with respect to the focal point, α0 is the linear absorption coefficient, R is the Fresnel reflection at the interface of the sample material with air, L (= l ) is the sample length, and  1− exp(−nα0l )  n n , I being the intensity at the focal point p0n = nγ (n+1)   (1− R) I0 0 nα0  

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p 0n = q 0 for a two photon absorption process with

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n=1. The values of the three-photon photon absorption coefficient γ obtained from rom the theoretical fit of the z-scan z data are 0.7 x 10-23 #$ ⁄% ,2.1 x 10-23#$ ⁄% , 3.6 x 10-23#$ ⁄% , for V2O5,3 mol% Na-doped V2O5 , 5 mol% Na- doped V2O5 respectively. The fitted transmittance curve for pure V2O5 and 3mol% and 5mol% Na-doped doped V2O5are shown in the figure 8a for input laser energy of 200 µJ As shown in figure 8b, the V2O5nanoparticles do not show significant OL threshold whereas in the case of Na-doped V2O5, the transmittance decreases sharply indicating strong OL effect at lower thresholds-ideal thresholds for practical applications. The limiting threshold for the 3 mol% Na-doped V2O5 was found to be 24.4 Jcm-2 which prolifically decreased to limiting threshold level of 1.18 Jcm-2in case of 5 mol% Na-doped V2O5.V2O5nanoparticles exhibited weak saturable absorption (SA) at low intensities and strong nonlinear attenuation of laser pulses at higher intensities due to the simultaneous contributions from NLA &nonlinear & scattering (NLS).. The presence of SA may be attributed to the presence of near resonant absorption arising from linear absorption of V2O5 nanoparticles near 532 nm excitation. excitation All our samples exhibited optical limiting performance due to significant signif NLS at higher intensities as shown in figure 8c. 8c The NLS in our samples plausibly arises from the formation of “microplasma “microplasma and microbubble” scattering centers in nanostructured samples [8].

Scan Data, Data b) OL Response, Figure 8: a) Overlay of Experimental Data and Theoretical Fitting of Z-Scan c) Nonlinear Scattering Data of V2O5, 3 Mol % Na-Doped V2O5, 5 mol % Na-Doped Na V 2O 5 Considering the fact that the sample is semitransparent at the excitation wavelength, the observed nonlinearity will have contributions from excited-state state absorption involving real excited states. The enhancement in the nonlinear absorption coefficient of V2O5 with increasing doping of concentration Na can also be attributed to the enhanced free carrier concentration. Hence, it is appropriate to consider the observed nonlinearity as an effective three-photon three process, where both genuine 3PA and sequential three-photon three photon absorption contribute to the phenomenon [3].

CONCLUSIONS In this study, we report the NLO absorption properties of vanadium pentoxide (V2O5) nanoparticles and Na-doped V2O5 nanoparticles. Open-aperture z--scan technique was employed to study the NLO absorption behavior of the synthesized samples using a second harmonic (532 nm) of Nd: YAG laser with 15 ns pulse width. The z-scan z data was fitted using the Matlab code to decipher er the mechanism of optical nonlinearity in V2O5 and Na-doped Na V2O5 nanoparticles. We found three-photon photon absorption to be the best fit for our data. The nanoparticles exhibited combined SA and RSA behavior at different input laser energies. From the study, we found that 5 mol % Na-doped Na doped V2O5 exhibited enhanced NLA



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compared to undoped V2O5 which we attribute to the enhanced free carrier concentration due to Na doping. The OL threshold of 5 mol% Na-doped V2O5 was found to be 1.18Jcm-2 with input laser energy of 200 µJ.

ACKNOWLEDGEMENTS Authors express their gratitude to Bhagawan Sri Sathya Sai Baba, the founder chancellor of SSSIHL, for his constant support and lab facilities. Authors also thank UGC and DST for the financial support.

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3.

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M. Sheik Bahae et al., 1990, “Sensitive Measurement of Optical Nonlinearities Using a Single Beam”, IEEE J. Quantum Electron. QE-26, pp. 760.

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8.

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