Silicon dioxide (SiO2), also known as silica, is an oxide of silicon (Si) that is found in nature in two different forms, namely amorphous and crystalline. Traditionally, amorphous SiO2 is used in many applications such as semiconductor circuits, microelectronics and optical fibers for telecommunication. In modern age research, amorphous glassy SiO2 has emerged as a potential host lattice for a variety of rare-earth ions to prepare light emitting materials or phosphors that can be used in different types of light emitting devices. SiO2 based phosphors can also be prepared by encapsulating semiconducting nanocrystals of zinc oxide (ZnO) and lead sulphide (PbS). In addition, recent studies have demonstrated that when semiconducting nanocrystals are incorporated in glassy SiO2 activated with trivalent rare earth ions (Ce3+,Tb3+,Eu3+,Pr3+) light emission from the rare-earth luminescent centres could be increased considerably as a result of energy transfer from the nanocrystals to the rareearths, i.e. semiconducting nanocrystals act as sensitizers for radiative relaxation processes in these centres.