Study On Preparation Of Cd_x/CdS-SiO₂Composite Film By Electrochemistry-sol-gel Method And Its Nonlinear Optical Properties

CdS based materials have excellent optical nonlinearities with easy preparation andstable performance, and have great prospect applied in optical communication andoptical information treatment area. It can be used in mine safety monitoring andinformation transmission. The electrochemistry-sol-gel method can combine theadvantages of both sol-gel method and eletrodeposition to easily produce films withhigh quality. In this dissertation, the same cadmium source and different sulfur sourcewere used to produce two kinds of sols. Cd_x/CdS-SiO₂composite transparent films wereprepared on ITO substrate by an electrochemically induced sol-gel method using theas-prepared sol as an electrolyte. By changing the sulfur source to control the form ofcadmium source in the sol (CdS or Cd²⁺), and then change the composition of films toimprove the third order nonlinear optical properties. The films’ microstructure andcomposition were characterized. The films’ optical properties and thickness were tested.And the third-order optical nonlinearity of films was analyzed. Results were obtained asfollows.⑴CdS-SiO₂composite sol was prepared with thioacetamide as sulfur source,cadmium nitrate as cadmium source and tetraethoxysilane as silicon source by sol-gelmethod. The electrochemical behavior of sol was studied by cyclic voltammetry. It wasproved that Cd²⁺had been converted into CdS by thioacetamide. And element of Cd insol entered the film in the form of CdS. Cd²⁺-SiO₂composite sol was prepared withthiourea as sulfur source, cadmium nitrate as cadmium source and tetraethoxysilane assilicon source by sol-gel method. It was proved that there was reduction reaction ofCd²⁺and codeposition of CdS by Cd（NO3）2and CS（NH2）2by cyclic voltammetry. Bychanging the sulfur source, composition and electrochemical behavior of the sols hadchanged.⑵The results of scanning electronic microscope-energy dispersive X-rayspectroscopy （SEM-EDS） showed that the two kinds of films were made of nano beamsof100nm×10nm and25nm×10nm. And the films were made of elements Si, Cd, Sand O. The results of X-ray diffraction （XRD） showed that there were CdS in the films.And there was Cd in the films produced by thiourea as sulfur source. Combining theresults of cyclic voltammetry, EDS and XRD it was proved that the film was CdS-SiO₂composite film produced by thioacetamide as sulfur source and the film was Cd/CdS-SiO₂composite film produced by thiourea as sulfur source. The composition ofthe films was successfully controlled by changing the sulfur source.⑶The Cd_x/CdS-SiO₂composite films had high transmittance, low reflectivity andweek absorption at400nm to1100nm. The thicknesses of CdS-SiO₂composite filmswere37.8nm to52.1nm and that of Cd/CdS-SiO₂composite films were48.0nm to310.5nm measured by profilometer. The thickness was related to electrochemicaldeposition conditions. The absorption coefficient of CdS-SiO₂composite films were2.39106m1to3.33106m1and that of Cd/CdS-SiO₂composite films were2.56105m1to3.59106m1.⑷The nonlinear refractive index, nonlinear adsorption coefficient and third ordernonlinear susceptibility of composite films were tested by Z-scan technique. TheCd_x/CdS-SiO₂composite films produced under different electrochemical conditions allhad features of nonlinear saturated absorption and self defocusing nonlinear refraction.The Cd_x/CdS-SiO₂composite films had third-order optical nonlinearity judged by theshape of open curves and close curves. In combination with transmittance, reflectivityand thickness the third-order nonlinear optical susceptibility （（3）） of CdS-SiO₂composite films were2.6510-6esu to5.4210-6esu and the （3）of Cd/CdS-SiO₂composite films were1.6210-6esu to1.0610-5esu calculated by theoretical formula ofZ-scan. The values stability reached or exceeded the highest reported in literatures. Thethird-order nonlinear optical properties of the films could be obviously improved bydoping Cd.⑸It was analysised that the third-order optical nonlinearity of Cd_x/CdS-SiO₂composite films came from CdS and Cd. Nonlinear mechanism of CdS was resonancenonlinearity and the nonlinear optical effect was strong. The element of S in CdS was indouble coordination and had two lone pair electrons without bonding. So CdS had fastnonlinear response. Metal of Cd could make film had optical nonlinearity by local fieldtheory and thermal resistance effect.In the dissertation sol-gel method and electrochemical deposition were combinedsuccessfully to prepare high transparent CdS-SiO₂composite films. And by doping Cdthe third order nonlinear optical properties were improved. The value of （3）could stablyreach10-6esu¹0-5esu. It was higher than the literature value of10-13esu¹0-7esu.