Study On Preparation Of Te/TeO₂-SiO₂ Composite Film By Electrochemistry-sol-gel Method And Its Nonlinear Optical Properties

Due to the excellent optical nonlinearity, TeOx based nonlinear optical material thin films have promising applications in optical communication, optical computer and optical information processing etc. For the preparation of their thin film, electrochemistry-sol-gel method takes the advantages of both sol-gel and electrodeposition methods, which help to fabricate high quality thin films of the composite materials. In this dissertation, using TeO₂-SiO₂ composite sol as electrolyte, Te/TeO₂-SiO₂ composite thin films was successfully prepared on the ITO glass by the electrochemical sol-gel method.α-TeO₂-Te/TeO₂-SiO₂ composite films were further obtained by thermo treatment of Te/TeO₂-SiO₂ composite thin films. In addition, third-order optical nonlinearities of as-prepared two kinds of composite films were studied by Z-scan technique.Firstly, TeO₂-SiO₂ composite sol was successfully prepared by using TEOS and Te(OPri)₄ as precursors. The as-prepared TeO₂-SiO₂ composite sols are very stable in ambient condition for long time, which make it easy for preparation of thin films by electrodeposition. The deposition mechanism of the composite film was then studied by using the cyclic-voltammetry (CV), Tafel plot and AC impedance techniques. The CV measurements showed that electrochemically induced sol-gel process included the electrodeposition of Te. The increase of local pH value in the anode area due to the evolution of hydrogen resulted in the formation of TeO₂-SiO₂ composite films. Because of the simultaneous electrodepositon Te occurred, Te/TeO₂-SiO₂ composite film finally formed. The Tafel plot was used to study the kinetic parameters of the deposition of Te, which showed the exchange current density i0 = 9.66×10^-7 A-cm^-2,and transfer coefficientα=0.149. Electrochemical AC impedance studies showed that the composite film could be formed in the cathodic potential range. The obvious dispersion effect in the AC impedance spectroscopy indicated the as-prepared thin films were of mosaic structure.The morphology and the composition of Te/TeO₂-SiO₂ composited films were then studied by scanning electronic microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. It was shown that Te particles with size of 100 to 300 nanometers were uniformly embedded in the as-prepared composited films. The EDX measurement showed that the Te/Si ratio in the composite films depended on deposition potential and time. High Te/Si ratio appeared at potentials about -0.8 V was high and relatively stable. However, when the potentials were more positive (-0.1 V and -0.4 V) or more negative (-1.5 V), the values of Te/Si ratio decreased. However, its value increased with the increase of deposition time. With the deposition time prolonging, the Te particles grew larger, and their shapes changed from spherical shape to hexagonal prism. XRD and TG measurements showed that Te particles were partially oxidized toα-TeO₂ during the thermo treatment in air, resulting in the formation ofα-TeO₂-Te/TeO₂-SiO₂ composite films.The nonlinear refractive index (γ), nonlinear adsorption coefficient (β) and third order nonlinear susceptibility (χ(3)) of composite films were tested by Z-scan technique. It was found both composite films belonged to self-defocused and saturated absorption materials.χ(3) of Te/TeO₂-SiO₂ composite films reached 10-14 (m/V)2 or 10-6 esu, and that ofα-TeO₂-Te/TeO₂-SiO₂ composite films are in the same order, which was much higher than that of tellurite glass.Both TeO₂ andα-TeO₂ embedded in the thin film consists of [TeO₄] units with trigonal bipyramid structure, which are the source of large nonlinear effect. In addition, the good conductivity of Te particles may also play important role in the excellent nonlinear property. On the one hand, the surface plasmon resonance absorption of Te particles may greatly enhance third-order nonlinearities due to the giant amplification of local electric field. On the other hand, there are a great amount of free electrons on/in metal particles surfaces, which move directionally under electromagnetic field and form strong electric dipole, resulting in strong polarization.