Optical limiting performance of ZnO nanoflakes and nanoplates embedded in PVA matrix

This paper describes the optical limiting performance of 2D ZnO nanoflakes and plates synthesized through a simple wet chemical method. Scanning electron microscopic imaging of these nanostructures revealed the shape evolution from the nanoflakes to nanoplates as the growth duration increased from 11 h to 18 h. The nonlinear absorption is studied using open aperture Z scan technique. The process behind the nonlinear absorption is predicted as two photon absorption and one photon assisted energy transfer to the nearby trapping sites. We observe the appreciable optical limiting threshold of 46.86 MW/cm for high pump power of 436 MW/cm for nanoplates compared with nanoflakes (169.49 MW/cm).


INTRODUCTION
The hunt for new nonlinear materials for optical limiters to protect sensors and eyes from the debilitating effects of lasers pulses has attracted great attention.Semiconductors are being experimented with this regard for the past few years because of their extensive applications in solid states electronics and optics.A number of research studies of optical limiting effects related to two photon absorption processes in semiconductor materials have been reported. 1,2,3Among these materials, ZnO is a key purposeful material exhibiting ultraviolet photoluminescence emission, transparent conductivity along with semiconducting, magnetic and piezoelectric properties; is a wide band gap semiconductor with band gap energy of 3.37eV, and large exciton binding energy of 60 meV at room temperatures.This high exciton binding energy of ZnO would allow for the excitonic transitions even at room temperature, which could mean high recombination efficiency for the spontaneous emission and low threshold voltage for the laser emission. 4The fabrication of 2D nanosized ZnO has been reported through a multi-step or long reaction procedure such as electrochemical method and thermal evaporation methods. 5ecent published works 6,7,8 have shown that nanoscale ZnO can display remarkable optical and electrical properties, which can be greatly manifesting its applications for near UV emitters and ultrafast UV modulators. 9Because the structure of ZnO, including the morphology, aspect ratio, size, orientation, and density of crystal have immense effects on its properties and applications and are usually influenced by the preparation techniques, researchers have prepared many special-shaped ZnO nano structures in order to fit various applications.So far, prismatic, 10 belt like 11 flower like 12 tubular, 13 tower like, 14 plates 15 like ZnO have been reported using physical and chemical techniques. 16owever, the assembly of nanoparticles in matrices is of the major interest in several optical and sensor applications 17 especially those require large area coating.The common approach to such materials includes casting of films using the mixture of nanoparticles and polymer and another is inbuilt growth inside the solid matrix.The former mode of fabrication of films is of interest in the present study.
Here, we describe a facile approach for controllable synthesis of ZnO nanostructures with flake and plate like morphologies.But to our knowledge, optical limiting studies of free standing polymer film incorporated with flakes-like plates-like ZnO structure synthesized through simple wet chemical method are the first time.These nanoplates and nanoflakes consist of forests of tiny crystals, millions of which would fit in a single square centimetre.This tiny forest absorbs radiation, making it potentially useful in the photonic device applications.
The optical limiting, which is governed by two photon absorption and that was studied by the Z-scan technique using an Nd: YAG laser (532 nm, 7 ns, 10 Hz), is widely used in material characterization due to its simplicity, high sensitivity and well-elaborated theory.

METHODS
All chemicals were purchased from Merck Ltd and used as received without further purification.The nutrient solution was prepared from an aqueous solution of zinc nitrate hexahydrate [Zn (NO 3 ) 2 6H 2 O] and hexamethylenetetramine (HMTA) [(CH 2 ) 6 N 4 ].The hexamine solution was added to the zinc nitrate solution dropwise while stirring.Finally the solution is kept undisturbed at 808C for 11 h and 18 h.The following reactions was involved in the crystal growth of ZnO. 18 The reaction decomposes HMT to formaldehyde (HCHO) and ammonia (NH 3 ), acting as a pH buffer by slowly decomposing to provide a gradual and controlled supply of ammonia, which can form ammonium hydroxide and support OH2 . 19Finally OH 2 anions react with Zn 2þ cations to form ZnO. To avoid sedimentation of nanocrystals, the solution was centrifuged and washed several times and finally embedded into the polyvinyl alcohol [ZCH2CH(OH)Z]n solution (15%) and developed the free standing films (thickness 80 -90 mm) (Plasto Mek, delta 0.2 kW, 230 V, 1 phase), this film is used as samples for the nonlinear studies.The thickness of the films was measured using a Mitutoyo Micrometer (series 193).Here PVA acts as the matrix for the homogeneous distribution and immobilization.By doing so optical properties of crystals can be utilized in more technological applications.
The size and morphology of ZnO samples were characterized by scanning electron microscopy (JEOL/EO, and JSM6390).The X-ray diffraction data were collected on an AXS Bruker D% diffractometer using Cu Ka-radiation (l ¼ 0.1541 nm, the operating conditions were 35 m A and 40 kV at a step of 0.0208 and step time of 29.5 s in the 2u range from 30 to 708.To determine the third order nonlinear optical characteristics of ZnO crystals such as nonlinear absorption, the single beam Z scan technique proposed and demonstrated by Sheik-Bahae et al., 20 was employed.The transmission of a laser beam that changes near the focal point during the sample translation along the propagation path through an open-aperture (OA) was measured.A Q-switched Nd: YAG laser (Spectra PhysicsLAB-1760, 532 nm, 7 ns, 10 Hz) was used as the light source.A 20 cm converging lens was used to focus the laser beam.The radius of the beam v 0 was calculated to be 35.4mm.The Rayleigh length, z 0 ¼ pv 0 2 /l, was estimated to be 7.4 mm, which is much greater than the thickness of the sample, and is an essential prerequisite for Z-scan experiments.The sample was fixed on a computer-controlled translation stage, so that it could be accurately moved through the focal region of the laser beam over a length of 6 cm.The transmitted beam energy, reference beam energy and the ratios were measured simultaneously using an energy ratio meter (Rj7620, Laser Probe Corp.) having two identical pyroelectric detector heads (Rjp735).The detected signals were acquired, stored and processed by a computer.Optical density filters were used to vary the laser intensity at the lens focus.

RESULTS AND DISCUSSIONS
In order to examine the surface morphology and for the dimension measurement, SEM was used.Typical SEM image of ZnO nano crystal and corresponding XRD pattern are shown in Figure 1.It can be seen that all diffraction peaks are caused by crystalline ZnO with hexagonal wurtzite structure (space group: P6 3 mc; a ¼ 0.3249 nm, c ¼ 0.5206 nm) is found to match with that mentioned in JCPDS 36-1451. 21When the heating time is prolonged for 11 h, the crystal growth along [0001] direction decreases, leads to the formation of nanoflakes, due to the sequential dissolution of the polar O [001I] and non polar [01I0] peaks. 22The d spacing along the (100) plane to be 2.807 A8.The lattice constants a and c were determined as a ¼ 0.3249 nm, c ¼ 0.5206 nm by analytical method. 23en heated for 18 h the ZnO nanoplates with uniform size have been fabricated in large scale.The direct band gap of the same is estimated from Tauc extrapolation method. 24he nonlinear absorption was measured from the normalized energy transmission using Z-scan without an aperture.The transmitted OA Z scan signal is given by Intensity (counts) Where L eff denotes the effective sample length, defined by with the linear absorption coefficient a and the sample thickness L.Where m is an integer, the parameter q 0 can be obtained by fitting the experimental results to the equation ( 5) where Z 0 and Z are the Rayleigh range and the translated length parallel to the beam propagation, respectively.I 0 denotes the intensity of the incident beam at the focal point.The Z scan traces are shown in Figure 2; fits of equation ( 5) to the experimental data are depicted in the figure by solid curves.
The experimental data shows a slight deviation from the theoretical curve which is fitted for two photon absorption.This deviation can be attributed to two photon induced free carrier absorption which is observed in particles with larger sizes. 2 One of the applications of reverse saturable absorption materials is in devices based on optical limiters, which are the devices that transmit light at a low input fluence, while they become opaque at high inputs.The Measured values of imaginary part of the third-order susceptibility (Im x3) and optical limiting threshold at a wavelength of 532 nm for different irradiation intensities are also tabulated in Table 1.As given in the table, the nonlinear response of the crystals depend on the pump power.As the power is increased, lowering of the nonlinear absorption coefficient occurs thereby diminishing the third order susceptibility, which can be due to the local field enhancement through the size-and structure-dependent interfacial interaction between the ZnO crystals, which influences the magnitude of the nonlinear absorption; and one photon assisted energy transfer from excited state to the nearby trapping site.
Reverse saturable absorption, which is generally allied with a large absorption cross section from excited state than the ground state, brings about optical limiting effects.This study observes that TPA governs the optical limiting in semiconductor materials.Semiconductor films with high TPA coefficients  and strong Kerr-induced nonlinear susceptibilities are very good candidates due to their small time response as optical limiters of intense short pulse radiation. 25The experimental investigation of the optical limiting in the samples was performed using OA Z scan at different input power density and concentration.
We have also studied the behavior of nano structures by changing the concentration of sample solution 1.5 m mol (C2) and 2 m mol (C3) tabulated in Table 2.There is no significant shift in the energy band gap evaluated from the optical absorption spectra.The increase of the pump power and the concentration leads to the enhancement of the optical limiting due to local field enhancement inside the plates and flakes clearly depicted in Figures 3 and 4.This property is desirable for the protection of sensors and human eyes from being affected by intense laser radiation.Therefore these films with immobilized semiconductor nanoplates and nanoflakes appear to be noticeable candidate for optical limiting applications.

CONCLUSIONS
The optical limiting performances of ZnO 2D nanostructures synthesized through the low-temperature wet-chemical method, which shows the shape evolution from the nanoflakes to nanoplates as the growth duration increased;were studied by Z scan method with nanosecond laser.We observe the appreciable optical limiting threshold of 46.86 MW/cm 2 for high pump power of 436 MW/cm 2 for nanoplates compared with nanoflakes (169.49MW/cm 2 ).The optical limiting enhancements in ZnO nanoplates and nanoflakes were mostly contributed from the interfacial interaction between the structure and the local-field enhancement; in addition to the one-photon-assisted-energy-transfer from the excited state to the nearby trapping sites.The studies shows that these materials are promising candidates for photonic and optoelectronic applications.

Figure 1 .
Figure 1.SEM images of ZnO nanocrystals (a-nanoflakes and b-nanoplates) and their X-ray diffraction patterns (nanoflakes is the upper one and nanoplates is the lower one).

Figure 2 .
Figure 2. Normalized transmittance of 1 m mol ZnO nanoplates/PVA composite film and nanoflakes/PVA composite film as the function of position for different input fluences in the open aperture scheme at 532 nm.The solid red line shows the theoretical fit.

Figure 3 .
Figure 3. Optical limiting response of ZnO nanoplates and nanoflakes for different Input fluences.

Figure 4 .
Figure 4. Optical limiting response of ZnO nanoplates and nanoflakes for different concentrations.

Table 1 .
Measured values of imaginary part of the third-order susceptibility (Im x3 * 10 210 esu) and

Table 2 .
optical limiting threshold of nanocrystals for various concentrations.