Abstract:
The photoluminescence (PL) of intrinsic defects and Cr$^{3+}$ impurity centers in nanosized alumina (Al$_2$O$_3$) crystals of various phase compositions ($\delta+\theta$ and $\alpha$ phases) has been studied by the method of time-resolved spectroscopy with vacuum-ultraviolet (VUV) excitation at $T$ = 7.5 and 295 K. It is established that the PL excitation spectrum and its kinetic parameters for $\alpha$-Al$_2$O$_3$ phase can change depending on the crystal grain size. In $\alpha$-Al$_2$O$_3$ samples with grain sizes below 100 nm, excitons localized on Cr$^{3+}$ impurity centers and surface-modified defects of the crystal structure are observed even at $T$ = 295 K. Manifestations of these exciton states in the PL spectra strongly depend on the grain size.
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