Dr. Bart Goris

EMAT, University of Antwerp, Belgium
Authors | Bart Goris, Marijn A. Van Huis, Sara Bals, Henny W. Zandbergen, Liberato Manna and Gustaaf Van Tendelooi.
Email |  sara.bals@ua.ac.be

ApplicationThermally Induced Structural and Morphological Changes of CdSe/CdS Octapods
AuthorsBart Goris, Marijn A. Van Huis, Sara Bals, Henny W. Zandbergen, Liberato Manna and Gustaaf Van Tendelooi.
JournalSmall 2012, 8, No. 6, 937–942
PublicationFull Publication Here – DOI: 10.1002/smll.201101897

Thermally Induced Structural and Morphological Changes of CdSe/CdS Octapods

ABSTRACT: Branched nanostructures are of great interest because of their promising optical and electronic properties. For successful and reliable integration in applications such as photovoltaic devices, the thermal stability of the nanostructures is of major importance. Here the different domains (CdSe cores, CdS pods) of the heterogeneous octapods are shown to have different thermal stabilities, and heating is shown to induce specific shape changes. The octapods are heated from room temperature to 700 °C, and investigated using (analytical and tomographic) transmission electron microscopy (TEM). At low annealing temperatures, pure Cd segregates in droplets at the outside of the octapods, indicating non-stochiometric composition of the octapods. Furthermore, the tips of the pods lose their faceting and become rounded. Further heating to temperatures just below the sublimation temperature induces growth of the zinc blende core at the expense of the wurtzite pods. At higher temperatures, (500–700 °C), sublimation of the octapods is observed in real time in the TEM. Three-dimensional tomographic reconstructions reveal that the four pods pointing into the vacuum have a lower thermal stability than the four pods that are in contact with the support.

Figure above: a,b) TEM images acquired from non-annealed CdSe/CdS octapods. Four pencil like ended pods and four flat ended pods can be observed. c, d, and e show visualizations of the 3D reconstruction of a non-annealed CdSe/CdS octapod. On these visualizations, the flat ended pods and the pencil-like ended pods are indicated as well as the 6-folded symmetry of the pods. f presents a HAADF-STEM projection of an octapod where the 6-folded symmetry is also visible. The high resolution STEM image in g shows that these 6 facets of the pods correspond to the {10-10} and {11-20} planes. h reports a high resolution STEM image of the cubic CdSe core of the octapod.
Figure above: a,b) TEM images of octapods upon in situ annealing to 300 °C. The pods of the octapod have become rounded. c) HAADF-STEM projection of octapod used to make a tomographic reconstruction. d) Visualization of the tomographic reconstruction confirming that all the pods have become rounded. From the slice through the reconstruction in figure e, it is clear that pure Cd segregates as droplets at the tips and the side of the pods. f) TEM image of an annealed CdSe/CdS octapod where one of these droplets is indicated with an arrow. The elemental maps of S (g) and Cd (h) confirm that the droplet contains pure Cd.
Figure left: Snapshots from in situ annealed CdSe/CdS octapods at a temperature of 600 °C. Rounding of the pods is observed before their sublimation.

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