Nanostructured metallic oxides coating

This project has been focused on structural, morphological, optical and photoluminescence properties of pure ZnO thin film. Here, zinc oxide thin films are grown on glass via facile and low cost sol gel spin coating process @ fixed spinning speed of 1000 RPM. The X-ray patterns, the transmittance, the particle size by the atomic force microscope are investigated. The particle size is equal to 120 nm and the surface roughness is found to be 23.33 nm. The photoluminescence analysis reveals near band emission and strong visible emission 2.11 and 2.80 eV.


Introduction
Nowadays, zinc oxide (ZnO) film is the most studied material due to its various properties such as high transmittance in visible range, direct band gap around 3.3 eV, high exciton binding energy of 60 meV [1].Zinc oxide was prepared by many techniques such as sol gel spin coating [2], spray pyrolysis deposition (SPD) [3][4], sputtering [5] and chemical vapor deposition CVD [6].We use the sol gel spin coating as film deposition technique because it is low cost, facile, rapid, unobtrusive and environmental process.All these prompted features of this multifunctional material, attract many researchers to use it in many applications like sensors, light emitting diodes, piezoelectric and catalytic devices.Here, ZnO nanostructures are achieved by facile sol gel route.ZnO nanostructures have been successfully prepared in several morphologies like nanorods and nanowires [7][8].This paper consists on detailed study of the structural, surface morphology, optical, photoluminescence properties of zinc oxide grown onto glass by a facile spin coating route.Furthermore ZnO nanorods were successfully synthesized and investigated by high resolution field emission transmission electron microscope (FE-TEM), electron diffraction, field emission scanning electron microscope (FE-SEM) and electron dispersive X-rays analysis (EDX) and atomic force microscope (AFM) analysis.

Experimental details
The ZnO films are produced by sol gel spin coating route.The substrate used is a microscope glass slides 76 x 26 mm supplied by object trager Isolab.0.5 Molar of dehydrated zinc acetate (Zn (CH3COO) 2 .2(H2O)), ( 99.5 %) supplied by Carlo Erba reagents, is dissolved in 10 ml of 2-Methoxyethanol stirred at 60°C for 10 mn and then 0.3 ml of the monoethanolamine (MEA) as stabilizer is added drop by drop, the clear solution is then obtained, the stirring continued for 1 hour.Consequently, the solution followed an ageing process for one day.Initially, the glass substrates were cleaned by a soft soap solution, washed systematically with the distilled water, then with ethanol in ultrasonic cleaner and finally were dried with argon.Using a micropipette the obtained gel was homogenously poured, on the substrate deposited on plates of spin coater (MTI, EQ-TC-100 desk-top type).The sample spins for one minute at 1000 RPM (rotate per minute); the sample is instantly heated at 150 °C for 10 mn.The process is repeated 5 times; finally the film is annealed at 400 °C for 1hour under air in furnace.In that way, the coated films are investigated by Shimadzu 3600 PC double beam UV-VIS-NIR spectrometer, the surface morphology is analyzed by the mean of field emission scanning electron microscope JEOL JSM 7001F FE-SEM, the films are also examined by transmission electron microscope JEOL JEM 2100F FE-TEM and the chemical analysis of films is given by electron energy dispersive X-ray (EDX) spectrometers.Furthermore, morphology is explored by atomic force microscope Park system XE-100E Non contact cantilever Si used 256x256 pixels.
Room temperature photoluminescence characterization is carried out using an experimental setup consisting of: a 325 nm 15 mW He-Cd laser (Kimmon), a 0.85 m double monochromator (SPEX, model 1404), and a GaAs photon counting photomultiplier (Hamamatsu).The range explored is from 350 to 600 nm, in 0.5 steps and a speed of 0.2 seconds/measured point.

Structural properties investigation
The X-ray diffraction pattern for zinc oxide film recorded at room temperature is shown in Fig. 1.The strong peak, well known (002) located at 2θ=34.42°, given by JCPDS card N° 36-1451 is displayed by a red dash line as depicted in figure 1.The as-grown films were identified as polycrystalline ZnO with a wurtzite crystal structure and preferred orientation along the (002) plane.As can be seen in X-rays spectra, film exhibits a polycrystalline structure and most of peaks are broadened.Since base of peaks are enlarged, the full width at medium height (FWMH) increases and grain size G, expressed as follows, is reduced [4]; Where K is constant estimated at 0.94, λ is the wavelength of the X-ray used 1.54 Å, β is the full width at half maximum (FWHM) which has maximum intensity and 2θ is the Bragg angle.According the (002) orientation, the grain size G is about 120 nm, the reticular distance d is of 0.26 nm and the textural coefficient is 1.1.

Microscopy observation
The AFM micrographs reveal that zinc oxide exhibit the nanorods which have grown according to z-axis direction as shown in figure 2

Optical Characterization
Figure 5 shows the dependence of transmittance T (%) and reflectance R (%) on incident photon wavelength which ranges within 200-2500 nm.T increases rapidly in UV spectrum, and reaches up to 86 % in visible range, as depicted inset of figure 5, and then varies slightly between 82 and 93% both in VIS and IR spectra.Reflectance attains 9.5 % at 384 nm, but it still minor in the whole photon wavelength range.The band gap energy of the ZnO films can be determined by the following relation, Where Eg (eV) is the optical band gap, α (m -1) is the absorption coefficient and ν (Hz) is the photon frequency.Figure 6 depicts how the direct energy gap of the coated film has been estimated by extrapolating the linear part of (αh)² plot versus photon energy to the wavelength axis.It found to be 3.26 eV, which agrees well with our previous result [1][2][3][4].

Photoluminescence analysis
The optical properties of the coated zinc oxide films nanorods are examined using photoluminescence at room temperature as shown in figure 7. PL spectrum exhibits various emission bands, including strong near band edge emission peak located respectively at 442 nm (2.80eV) which corresponds to blue emission.This emission peak may prove that ZnO nanorods have high cristallinity.Others peaks of low intensity located at 447nm -538nm (2.77eV-2.30eV),which correspond respectively to green band emission.These bands might due to defects occurrence such as oxygen traps.ZnO exhibits the emission peaks with differents energies

Conclusion
Zinc oxide nanorods were successfully synthesized by facile sol gel spin coating route.It reveals that zinc oxide was present in high amount in coated film by XRD pattern and EDX analysis.These ZnO nanorods were high transparent in VIS and IR ranges.SEM and TEM images reveal clusters presence at nanoscale, and the Honeycomb architecture is observed at high magnification.Stron visible emissions are detected by PL investigation.

Figure 3 .
Figure 3.A high-resolution TEM image and an electron diffraction pattern obtained for coated sample ZnO produced at 1000 RPM ( top), bright field TEM image (bottom).

Figure 5 .
Figure 5. Transmittance ( balck curve) and Reflectance ( red curve) plots against photon wavelength of spin coated ZnO film, inset shows the transmittance in visible range.

Table 1 .
EDX analysis results of ZnO film produced @ 1000 RPM.