Optical properties of thin anodic alumina membranes formed in a solution of tartaric acid

Anodic alumina membranes were obtained in an aqueous solution of tartaric acid at constant current density. An integrated investigation of the effect of heating temperature on optical properties and its correlation with structural transformation of anodic alumina has been performed. In the case of the as anodized membrane, the optical band gap is about 3.25 eV. With growing heating temperature of the samples from 500 to 700 °C the band gap increases from 3.35 to 3.5 eV. As the value of the optical gap is much less then for the anodic alumina (about 6.5 eV) it was concluded that the absorption of the light of energy of 3.25-3.5 eV is not fundamental and is due to the presence of impurities. By Fourier-transformed infrared and photoluminescence spectroscopies the possibility of formation of partially oxidized amorphous carbon was shown in the as-anodized and heat treated membranes. The model for incorporation of oxidized amorphous carbon was also suggested. Similar to the a-C:H the optical properties of the tartaric acid anodic alumina are determined by the fraction of sp2-sites in sp3-matrix. The increase in the impurity gap value with increasing heating temperature is believed to be due to ordering in the structure of oxidized amorphous carbon.