OPTICAL CHARACTERISTICS OF Cr2O3/Al2O3 COMPOSITE STRUCTURE

Abstract

The absorption, luminescence excitation, and photoluminescence (PL) spectra were studied for the nanocomposite chromium oxide (Cr2O3)/aluminum oxide (Al2O3) structure in the crystalline state produced by annealing pre-impregnated anodic alumina chromium anhydride at high temperatures. A PL band with maximum intensity at 615 nm, which is not characteristic of bulk materials, was found in a Cr2O3/γ-Al2O3, structure annealed at Тa = 900°С. This band was attributed to hexavalent chromium (Cr6+) ions formed as a result of trivalent chromium (Cr3+)-Cr6+ redox processes on the surface of Cr2O3 crystallites at annealing temperatures of 400-900°С. The emergence of an additional excitation band of Cr6+ ions at 520 nm was explained, with good agreement with the experimental results. It is shown that the photoluminescent properties of the nanocomposite Cr2O3/γ-Al2O3 structure are different for different ambient temperatures: at liquid nitrogen temperature (Т = 77.4 K), two broad bands were observed at 500 and 670 nm, while there was no PL at room temperature. For the nanocomposite Cr2O3/α-Al2O3 structure annealed at 1300°C, the photoluminescent properties were caused by radiative transitions in the structure of the energy levels of the Al2O3 defects of the anion-vacancy type and by impurity centers. In addition, the R-line half-width of Cr3+ for the nanocomposite Cr2O3/ Al2O3 structure was ~ 250 cm-1, which was larger than ~ 100 cm-1 for the original anodic alumina. This may be due to the greater defectiveness of the crystal lattices. Also, for the Cr2O3/Al2O3 structures, a significant increase in absorption in the ultraviolet range was found compared to the original Al2O3.