Thermal decomposition of single source precursors and the shape evolution of CdS and CdSe nanocrystals

A systematic analysis of the thermal decomposition of various single-source precursors is reported with the aim of finding a correlation between the pattern of thermal decomposition and morphology of the nanocrystals resulting from the thermolysis of these precursors in a coordinating solvent. The precursors studied are cadmium complexes of N,N′-dioctylthiourea, N,N′-diocyclohexylthiourea, N,N′-diisopropylthiourea, N,N′-tetramethylthiourea, dithiobiurea, ethylxanthic acid, thiosemicarbazide, selenosemicarbazide. Cadmium complexes of thiosemicarbazide and selenosemicarbazide uniquely yield rod-shaped CdS and CdSe nanocrystals respectively while all other precursors yield spherical CdS nanoparticles. Nanorod formation without the aid of any external shape-directing agent is explained through analysis of the thermal decomposition patterns, as observed by thermogravimetric analysis, for the range of precursor molecules. It is suggested that the low activation energy for the semicarbazide precursor decomposition, compared to those that produce dot-shaped nanocrystals, provides conditions favourable for the growth of nanorods. Evidence supporting the idea that the semicarbazide precursors furthermore release a structure-directing agent during decomposition is provided by infrared spectra and elemental analysis. Hence it can be presumed that thiosemicarbazide and selenosemicarbazide ligands each act as both the source of sulfur and a shape-directing agent.