We report the effect of hydrogen on the crystallization process of silicon nanocrystals embedded in a silicon oxide matrix. We show that hydrogen gas during annealing leads to a lower sub-band gap absorption, indicating passivation of defects created during annealing. Samples annealed in pure nitrogen show expected trends according to crystallization theory. Samples annealed in forming gas, however, deviate from this trend. Their crystallinity decreases for increased annealing time. Furthermore, we observe a decrease in the mean nanocrystal size and the size distribution broadens, indicating that hydrogen causes a size reduction of the silicon nanocrystals. ... Read more

We demonstrate an analytical method to optimize the stoichiometry and thickness of multilayer silicon oxide films in order to achieve the highest density of non-touching and closely spaced silicon nanocrystals after annealing. The probability of a nanocrystal nearest-neighbor distance within a limited range is calculated using the stoichiometry of the as-deposited film and the crystallinity of the annealed film as input parameters. Multiplying this probability with the nanocrystal density results in the density of non-touching and closely spaced silicon nanocrystals. This method can be used to estimate the best as-deposited stoichiometry in order to achieve optimal nanocrystal density and spacing after a subsequent annealing step. ... Read more

This paper studies photochemical upconversion in solutions of octaethyl porphyrin (OEP) and diphenyl anthracene (DPA). The system has been widely used as a standard model system in the field of photochemical upconversion. Although, the kinetics of elementary processes contributing to photochemical upconversion in it have been extensively studied, there has been no research on the efficiency of upconversion in the system, despite of the fact that this parameter is detrimental for potential applications of photochemical upconversion process. We determine the yield of photochemical upconversion in a number of metal based OEP/DPA systems. Additionally, we studied the dependence of kinetic, and efficiency parameters of the process on the core metal of the porphyrin. We showed that the overall efficiency of photochemical upconversion depends significantly on the core metal of the triplet sensitizer porphyrin molecule. We attribute this effect to the differences in efficiency of triplet energy transfer from metal based OEP to DPA depending on the core metal. ... Read more