Structure and thickness dependent optical properties of nanocrystalline haematite thin films

An aqueous sol-gel process has been developed to deposit nanocrystalline haematite (alpha -Fe2O3) thin films by dip-coating. Stable, positively charged colloidal sol particles of hydrous iron (III) oxide with an average particle size (Z(av)) of 6 nm were prepared and coated onto quartz plates under controlled conditions. The sols were characterised using photon correlation spectroscopy (PCS) for Za, and size distribution. The re-dispersible gel was characterised by thermoanalytical techniques (TG-DTA) and also by isothermal heating followed by X-ray diffraction to identify the reaction pathway to form alpha -Fe2O3. The films as deposited were X-ray amorphous up to 450 degreesC, nan degrees Crystalline in the range 450 to 600 degreesC, crystalline and single phase above 600 degreesC. The films were optically transparent, continuous and had uniform thickness in the range between 180 nm to 1000 nn depending on deposition conditions. The optical band gap varied from 2.1 to 2.6 eV as a function of film thickness. The refractive index increased with increase in annealing temperature but decreased with film thickness.