Effect of lanthanum doping on the structure and optical properties of nanocrystalline vanadium pentoxide films prepared by sol–gel method

The structural and optical properties of lanthanum oxide doped nanocrystalline vanadium pentoxide films with the chemical composition xLa2O3-(1-x)V2O5.nH2O (where x = 0.25, 0.50 and 1.0 mol%) prepared by sol–gel method were studied. The XRD analysis also revealed that the (002) line is noticeable in the pure film and gets sharper by the addition of Lanthanum, which indicates a layer of intercalation between the vanadium layers. The average crystallite size decreased with increasing Lanthanum content from 4.45 nm to 3.57 nm. By using double-beam UV–VIS spectrophotometers, the optical properties were studied by measuring the absorption, reflectance and transmittance of the prepared films. Some optical parameters like absorption coefficient α, dispersion energy parameters, refractive index n, optical band gap Eop for various transition mechanisms, real parts and imaginary part of the dielectric constants and effective mass were calculated. The absorption coefficient slightly increases with increasing La content, which can be attributed to the increasing of lattice distortion as a result of crystallite size increasing as indicated in the XRD. The transition mechanism was found to be indirect allowed type with optical band gap Eop increasing relative to the La content. By assuming hydrogen like model, the carrier’s contents N were deduced. The absorption spectrum behavior in visible and UV region suggests a promising solution for solar cells and optical-electronic applications.