Photoexcitation of local surface structures on strontium oxide grains

The photoluminescence properties of strontium oxide grains result from the photoexcitation of local surface structures and are characterized by intense emission bands in the visible light range. Using diffuse reflectance and photoluminescence spectroscopy, we have investigated SrO nanocrystals obtained by chemical vapor deposition (CVD) and subjected to thermal activation under high vacuum conditions afterward. Transmission electron microscopy reveals that compact and morphologically ill-defined SrO grains with sizes up to 200 nm arise from the coalescence of various misaligned nanocrystals that aggregate and intersect in the course of thermal treatment. However, despite the low specific surface area of less than 1 m(2) g(-1), intense PL emission bands can be induced by selective excitation of surface anions in (100) planes, edges, and oxygen-terminated corners. Transfer of excitation energy across the SrO surface from sites of higher coordination to those of lower coordination as reported by Coluccia (Stud. Surf. Sci. Catal. 1985, 21, 59) has also been observed for CVD SrO. Comparison of room temperature measurements with those carried out at T = 77 K reveals a strong temperature dependence of all PL emission effects as well as the presence of a specific radiative deactivation process that is attributed to 4-fold coordinated surface sites and subjected to thermal quenching at room temperature. It was found that the abundance of excitation and emission sites in the surface region of the polycrystalline SrO grains can be controlled by the annealing time. Prolonged thermal treatment at 1170 K depletes low-coordinated surface elements and induces the enhancement of 5-fold coordinated surface anions located in extended (100) planes.