Publication date: 27th June 2014
Occurrence of polytypism in II–VI and III-V semiconductors has been broadly observed due to the stacking faults between lattice planes. Zinc Blende and wurtzite are the two very commonly observed polymorphs. Due to low energy difference between these polymorphs, switching between them can be possible either kinetically or by selective ligand attachment. This allows the tuning and fabrication of complex shaped nanocrystals. Since the modification in crystal structures induces changes in optical, electronic and other properties, these polytypic nanocrystals, having engineered structural faults, are expected to provide new possibilities for photovoltaic and optoelectronic devices. In colloidal based approaches, binary chalcogenide nanocrystals such as CdSe and CdTe are well known examples of linear and branched polytypic II-VI semiconductors. However, ternary and quaternary copper-based chalcogenides are more complex systems for phase control due to their intense atomic organization, involvement of various ligands in synthesis and wide range of composition variation. Recently, linear polytypism in CZTSSe1 and branched polytypism in CTSe2 and CCTSe3 has been reported, giving a new direction to the shape and phase controlled multicomponent copper chalcogenide nanocrystals.
Here, we have explored various possibilities of formation of different shaped linear and branched polytypic Cu2ZnSn(SSe)4 nanocrystals via colloidal synthesis. In our studies, we designed synthetic routes to form novel morphological structures of polytypic Cu2ZnSn(SSe)4 nanocrystals by different approaches such as ligand variation, phase transformation and different metal precursors- (i) Cu2ZnSn(SSe)4 monopods having a pyramidal shaped zinc blend core with a wurtzite arm emerging from one of the faces, (ii) Cu2ZnSn(SSe)4 rugby shaped nanocrystals two zinc blende-derived ends and one wurtzite-derived centre part, (iii) Cu2ZnSn(SSe)4 bullet shaped polytypic nanocrystals via phase transformation of wurtzite Cu2ZnSn(SSe)4. The synthesis of these different polytypics has been tried on various Se/S compositions. The effect of different reaction parameters has been extensively studied and focus has been given on the formation mechanism of polytypes. The nanocrystals have been characterized by TEM, STEM, EDX, SEM, XRD, UV-vis NIR techniques. These polytypic nanostructures are being expected to open up new avenues to band-gap alignment and engineering.
References:
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2. Wang, J. et al, J. Am. Chem. Soc., 2013, 135, 7835.
3. Zamani, R. R. et al, ACS Nano, 2014, 8, 2290.
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