Identifying and Mapping the Polytypes and Orientation Relationships in ZnO / CdSe Core Shell Nanowire Arrays

Abstract

Core shell ZnO nanowire (NWs) heterostructures have emerged, over the past decade, as a potential building block for a large variety of nanoscale optoelectronic devices, including self‐powered UV photodetectors, dye‐sensitized solar cells (DSSCs), and extremely thin absorber (ETA) solar cells. These heterostructures benefit from a high absorption over the UV and visible parts of the electromagnetic spectrum through sophisticated optical processes ( i.e ., optically guided and radiated modes). In this work, we focused on ZnO / CdSe core shell NW heterostructures. The ZnO NWs are typically grown by chemical bath deposition on top of a ZnO seed layer deposited by sol‐gel process and strongly oriented along the polar c‐axis, but with no in‐plane orientation. Then, the growth of the CdSe shell was performed by molecular beam epitaxy. CdSe is known to crystallize into the two following polytypes: cubic zinc blende (ZB) and hexagonal wurtzite (WZ). Identifying the different polytypes of CdSe by standard characterization techniques, such as x‐ray diffraction (XRD) and selected area electron diffraction (SAED) using transmission electron microscopy (TEM), as well as Raman scattering, is complicated in the present case. Both the ZB and WZ crystalline phases have very similar properties: most of the diffraction peaks and phonon modes of the ZB crystalline phase are located very closely to those of the WZ crystalline phase; similarly, the bang gap energy of both crystalline phases is almost identical. In addition to the identification of the different polytypes and potential epitaxial relationships, the spatial information on the local scale through mapping is of great interest, but requires the use and development of advanced TEM‐based experiments. In order to address these issues, the morphology and structural properties of ZnO / CdSe core shell NW heterostructures are thoroughly investigated by field‐emission gun scanning electron microscopy (FEG‐SEM), XRD, Raman spectroscopy, TEM‐HRTEM (Figure 1) and ASTAR (Automated crystal phase and orientation mapping in TEM). We show the strong interest in ASTAR for identifying and mapping the different polytypes of the CdSe shell , but also for revealing the occurrence of orientation relationships with the ZnO NWs, as shown in figure 2.