Investigation of some physical properties of ZnO nanofilms synthesized by micro-droplet technique
Résumé
In this paper, ZnO nanocrystals were synthesized using a simple micro-droplets technique from a solution prepared by dissolving zinc acetate di-hydrate [Zn(CH 3 COO) 2 , 2H 2 O] in methanol. Microdroplets were deposited on glass substrates heated at 100 °C, the obtained samples of ZnO films were investigated by XRD, AES, AFM, ellipsometry and PL. XRD patterns reveal the wurtzite structure of ZnO where the lattice parameters a and c, calculated from XRD signals, show a nanometric character of ZnO nanoparticles. The chemical composition of ZnO film surfaces was verified by Auger electron spectroscopy (AES). From Auger signals, oxygen (O-KLL) and zinc (Zn-LMM) Auger transitions indicate well the presence of Zn-O bonding. The surface topography of the samples was measured by atomic force microscopy (AFM) where ZnO nanoparticles of average size ranging between 20 and 80 nm were determined. Some optical properties as dielectric constants, refractive index, extinction coefficient as well as the optical band gap were determined from ellipsometry analysis. The dispersion of the refractive index was discussed in terms of both Cauchy parameters and Wemple & Di-Dominico single oscillator model. The photoluminescence (PL) measurements exhibited two emission peaks. The first at 338 nm, corresponding to the band gap of ZnO, is due to excitonic emission while the second around 400 nm, is attributed to the single ionized oxygen vacancies. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction Zinc oxide (ZnO) is a material which is very familiar to scientists because it is well known for its excellent physical properties as ultra violet absorbance, piezoelectricity and luminescence at high temperatures. We can count thousands of papers dating back as early as 1935 [1]. More recently, ZnO has again entered the scientific spotlight, this time for its semiconducting properties. ZnO is a semiconductor with interesting characteristics such as a wide and direct bandgap (3.37 eV at room temperature) and a larger exciton binding energy (60 meV) [2,3]. It is a very interesting material for its large applications in many technological fields for its useful properties (optical, electrical, mechanical, chemical.. .). A considerable research interest in ZnO was performed due to many potential applications particularly in short wavelength for optoelectronic devices operating in the blue and ultraviolet (UV) region such as light-emitting diodes (LED) [4] and gas-sensing applications [5]. Today, the research is focused on the nanostructured ZnO because there is a high surface-to-volume ratio and show enhanced chemical stability and electrical performances [6,7]. Several chemical and physical methods have been used for the preparation of ZnO thin films nanostructured such as MOCVD [8], chemical vapor transport (CVT) [9], spray pyrolysis [10–12], sputtering [13], laser ablation [14,15]. The purpose of this paper is to provide a simple chemical synthesis route of ZnO nanofilms. This technique called ''micro-droplets technique " is low cost and easy to implement. It is based on the deposit of microdroplets on heated glass substrates. The detail of this technique is described in the following paragraph. Experimental part ZnO nanofilms preparation ZnO nanofilms were obtained by micro-droplets route. The principle of this method is to prepare a solution by dissolving 10 À2 M of zinc acetate di-hydrate [Zn(CH 3 COO) 2 , 2H 2 O] in a beaker containing 150 ml of methanol. Microdroplets of 0.05 ml of volume http://dx.
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