Investigation of the effects of acidic pH and annealing on the properties of nanostructured TiO2 thin films for waveguiding applications

Abstract

In this work, titanium dioxide (TiO2) thin films are synthesized by the simple sol-gel process using the dip-coating technique at various pH values (pH 6, 4 and 2). The influence of acidic pH and annealing at 500 °C in air for one hour on the microstructure, morphology, optical and waveguide properties of the prepared thin films are investigated carefully using various characterization techniques. X-ray diffraction (XRD) and Raman spectroscopy (RS) analysis show that all the annealed films are polycrystalline and possess only anatase crystal structure with a preferential orientation along the (101) plane. The intensity of the diffraction and Raman peaks as well as crystallite size were found to increase with increasing sol acidity which indicates an improvement in the crystallinity of the films. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images revealed that morphology, grain size and surface roughness of the films are influenced by heat treatment and pH of the sol. According to UV–Visible measurements, all the TiO2 thin films are highly transparent with an average transmittance of more than 74% in the visible region. It is also observed that the optical transmission and energy band gap decrease with sol acidity and annealing. M-lines spectroscopy (MLS) measurements at a 632.8 nm wavelength put into evidence that TiO2 planar waveguides support single confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. Moreover, both TE and TM refractive indexes and the birefringence of the anatase films are found to increase with acidity of the sol.