MS-8-P-3337 Electron tomography and aberration-corrected TEM and STEM study of the polarity of colloidal wurtzite CdSe nanopyramids used in assemblies and as seeds for CdS pod growth

Several protocols have been reported in the last few decades for the production of colloidal CdSe nanocrystals (NCs) with the wurtzite crystal structure. In particular, NCs elongated along the c-axis were subjected to studies both aimed at calculation of equilibrium configurations (e.g., [1]) and at the experimental determination of the polarity (e.g., [2]). These results report of NCs often showing different endings at the opposite <0001> directions, with flat ends pointing towards the [0001] direction. This is due to the preferential binding of the most common surfactants to the Cd ions and to the consequent higher stability of (0001) facets, terminating with Cd atoms exposing one dangling bond.
In this contribution, CdSe NCs were obtained using a mixture of two different Cd precursors (CdCl₂ / CdO=50% wt.) and a lower amount of phosphonic acids (hexylphosphonic acid and octadecylphosphonic acid) compared to the standard CdSe nanorod synthesis. The synthesis results in monodispersed wurtzite CdSe nanopyramids (NPs, Fig. 1) with a {0001} equilateral base facet (side length = 20 nm) and three lateral {01-12} isosceles sides (lateral edges = 16 nm). The NP shape is directly evidenced by HAADF tomography volume reconstruction (Fig. 2a). Preliminary results of aberration-corrected HRTEM imaging in negative C_S conditions [3] and exit wave reconstruction (EWR) show phase variations in few Cd-Se dumbbells (Z_Cd=48, Z_Se=34), suggesting the NP base as corresponding to (000-1) planes (Fig. 1 b-c). These analyses were carried out on an image-corrected Jeol JEM 2200FS microscope operated at 200 kV. A further preliminary proof comes from annular bright field (ABF) imaging, obtained on a probe-corrected FEI Titan “cubed” microscope (operated at 200 kV, see Fig. 1d).
The unusual polarity of these CdSe NPs is mainly related to the use of CdCl₂ as a Cd precursor, as Cl^- is known to form complexes with phosphonic acids [4], therefore limiting their amount for NC surface passivation. This effect is also enhanced by the relatively low amount of phosphonic acids employed during the process. The detailed knowledge of the faceting of these peculiar NCs will be useful to elucidate the seeded growth of wurtzite CdS branches starting from the NPs (Fig. 2b). The knowledge of the polarity of the NP base will be fundamental also to explain the formation of peculiar patterns based on these NCs upon addition of particular surfactants (Fig. 2c).

[1] L. Manna et al., J. Phys. Chem. B 109, 6183 (2005)
[2] G. Bertoni et al., ACS Nano 6, 6453 (2012)
[3] K. Urban et al., Phil. Trans. R. Soc. A 367, 3735 (2009)
[4] M. R. Kim et al., ACS Nano 6, 11088 (2012)