Colloidal semiconductor nanocrystals play a key role in the quest for novel functional materials with enhanced optical, electrical and physical properties. In particular, copper based ternary and quaternary chalcogenides such as CuInS₂ (CIS), CuInGaS₂ (CIGS) and Cu₂ZnSnS₄ (CZTS) are attracting increasing interest as next generation photon absorbing layers due to their high energy conversion efficiencies, high optical absorption coefficients, compositionally tunable band gaps and relatively low toxicity._1,2 In comparison to the well-understood binary II-VI systems, where precise size and shape control over the synthesis has become routine, the difficulty in balancing the reactivity of three, four or even five different precursors, to synthesize ternary, quaternary and quinary materials respectively, severely limits comparable control. To date, only one report exists on the formation of CuInGa(SSe)2 (CIGSSe) nanocrystals by a colloidal based approach, in which 0D nanocrystals have be achieved in the tetragonal phase.₃

Herein, we conduct a comprehensive study on the synthesis of CIGSSe nanocrystals and achieve compositional, structural and crystal phase control of CIGSSe nanocrystals through optimization of the ligand chemistry. We achieve, for the first time, the formation of 2D nanoplates in the metastable hexagonal wurtzite phase, with the ample chemical and structural freedom in this system permitting band gap tuning through variation of the chalcogen ratio along with no drastic changes in the nanoplate morphology. Interestingly, we also observe the formation of an unusual ‘walnut’ shaped morphology which is the first of its kind in copper chalcogenide based systems. The study was also extended to investigate a series of injection temperatures, precursors, nature of solvents and ultimately understand how seemingly minute changes in the reaction can significantly alter the nucleation and growth kinetics. The structure, shape and composition of these nanomaterials were subsequently investigated with transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and UV-vis-NIR techniques.

(1) Repins, I.; Contreras, M. A.; Egaas, B.; DeHart, C.; Scharf, J.; Perkins, C. L.; To, B.; Noufi, R. Progress in Photovoltaics: Research and Applications 2008, 16, 235.

(2) Pan, D.; Wang, X.; Zhou, Z. H.; Chen, W.; Xu, C.; Lu, Y. Chemistry of Materials 2009, 21, 2489.

(3) Chang, S.-H. ; Chiang, M.-Y. ; Chiang, C.-C. ; Yuan, F.-W. ; Chen. C.-Y. ; Chiu, B.-C. ; Kao, T.-L. ; Lai, C.H. ; Tuan, H.-Y. Energy and Environmental Science 2011, 4, 4929