In this paper we summarize our recent results on the structural and optical properties of silicon nanoclusters (nc). The structural properties have been investigated by energy-filtered transmission electron microscopy, allowing one to determine the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts. Moreover, devices based on both amorphous and crystalline Si nc are demonstrated. These devices are extremely stable and robust, resulting in an intense electroluminescence at around 900 nm. Finally the integration of photonic crystals with Si nc light-emitting diodes will be demonstrated.