We present the optical properties of rare earth (RE)-doped $\rm{Al_{2}O_{3}}$ thin films and discuss their possible use in applications like gate dielectric material and in coating industry. Aluminum oxide films doped with RE elements such as Ce, Nd, Gd, and Dy are synthesized on glass substrate using ultrasonic spray pyrolysis technique at 400$^{\circ}$C. The concentration of rare earth element is varied from 0.5 to 5 mol% in 0.1 M solution of $\rm{Al_{2}O_{3}}$. The X-ray diffraction analysis indicates that the thin films deposited with and without rare earth doping have an amorphous structure. Further, the optical properties of RE-doped $\rm{Al_{2}O_{3}}$ thin films are studied by using UV–visible spectroscopy and photoluminescence measurement. The band gap is found to be 4.06eV for $\rm{Al_[2}O_{3}}$ thin film. A small blue shift is seen in the optical spectra of RE-doped samples as compared to undoped $\rm{Al_[2}O_{3}}$ film. Dielectric constant of alumina thin film increases with doping of Gd and Dy while it decreases with Ce and Nd doping. Concentration quenching effects are observed in the photoluminescence spectra of Ce, Nd, Gd, and Dy-doped $\rm{Al_[2}O_{3}}$ films. Among all these RE-doped $\rm{Al_[2}O_{3}}$ thin films, Gd and Dy-doped $\rm{Al_[2}O_{3}}$ films exhibit a potential for the construction of dielectric gate in transistors or as a coating material in the semiconductor industry.

In the present work, rare earth (RE)-doped PbO thin films are characterised for their structural, morphological, electrical, optical and gas sensing properties. Thin films of PbO and Ce-, Nd- and Eu-doped PbO are deposited on a glass substrate using ultrasonic spray pyrolysis technique. We have considered 1, 5 and 10 mol% concentration of RE elements in PbO. X-ray diffraction patterns confirmed the formation of polycrystalline PbO and RE-doped PbO thin films with tetragonal structure. Scanning electron micrographs and optical absorption spectra of RE-doped PbO thin films revealed the decrease in average particle size and band gap with an increase in RE dopant concentration in PbO. The band gap of undoped PbO thin film is 2.58 eV. The concentration-dependentvariations in the band gap of RE:PbO thin film are observed. The band gap of RE:PbO varies from 1.75 to 3.05 eV. At room temperature, the RE:PbO systems are found to be highly sensitive towards a wide concentration range(333 to 3330 ppm) of CO$_2$ compared to the undoped PbO film. Ce- and Nd-doped PbO thin films show enhanced sensitivity towards CO$_2$ gas compared to Eu-doped PbO thin films. At room temperature, significant increase inthe gas sensor response of doped samples have been corroborated to their increased surface area, morphological changes and oxygen vacancies present.