The present invention relates to a method of forming a core/shell nanocrystal of semiconductor material. Typically the core may comprise CdTe and the shell may be CdS. The shell is synthesised on the core in an aqueous solution. In the method, the previously synthesised cores are placed in the aqueous solution, reactants that form the shell and a thiol such as 3-mercaptopropionic acid (MPA) are added, and the mixture is refluxed until the completion of the shell at the desired thickness. The synthesis of the shell is aided by the provision of an interface zone between the shell and core so that lattice mismatch between the core and shell is reduced. The interface zone may be produced using a method that provides a gradient alloyed core with increased levels of sulphur, for example, at the surface relative to the centre of the core. Alternatively the interface zone may be a separate layer on a homogenous core.