Spontaneous Polarization induced Asymmetry (SPA) Growth

by Daniel Moore

    The most profound phenomenon of the nanosaw structure is the growth of asymmetric saw teeth on one side and the other side is straight and smooth. From the structure model shown in Figure 1a, the (0001) surface of wurtzite can terminate with different atom plane. Take CdSe as an example, the (0001) surface is terminated with Cd and the (000-1) is terminated with Se (Figure 1a). To quantitatively determine the polarity of the surfaces, convergent beam electron diffraction (CBED) pattern was recorded from the ribbon, as given in Figure 11b. The intensity distributions in the (0002) and the (000-2) disks are significantly different, which is due to the non-central symmetric structure of CdSe, which can uniquely determine the polarity of the CdSe nanosaws. Quantitative interpretation of the CBED relies on dynamic simulations, which were performed using an improved version of the Bloch wave program. A comparison of the experimental pattern with a theoretically calculated pattern (Figure 1) indicates that the saw teeth grow out of the (0001)-Cd surface. Therefore, the growth of the saw teeth is due to the catalytically active Cd-(0001) surface, while the Se-(000-1) is relatively inactive, producing the asymmetric growth morphology.

Figure 1. (a and b) TEM images of the CdSe nanobelts, showing that the belts are dominated by the wurtzite structure rather than the zinc blend structure; (c and d) high resolution TEM images showing the existence of the zinc blend phase near the root of the teeth that is several atomic layers thick.

    The most dominant morphology found in samples that exhibit spontaneous polarization-induced asymmetric (SPA) growth is the “saw” shape nanobelt, with one side flat and the other side sharp “teeth.” Detailed study of CdSe has led to some interesting discoveries relating to the growth of the saw and teeth structures. As stated previously, many of the bulk or thin films of II-VI semiconductors can exist in the typically more stable zinc blende structure, but most of the nanobelts and nanosaws presented here are dominated by the Wurtzite structure (Figure 1 a and b). High-resolution TEM image recorded near the root of the nanoteeth of CdSe indicates the existence of a different phase, which is several atomic layers in thickness and corresponds to the zinc blende structured CdSe (see Figure 1c). The zinc blende is a result of changing the stacking sequence from ABAB for the hexagonally structured wurtzite to ABCABC. The energies for the two phases are quite close and it is relatively easy to stimulate phase transformation. Analogous structure has been also reported for ZnS. The magnitude in morphology oscillation for the saw structure is much lower than that for the nanoteeth. High-resolution TEM from the saw region found neither planar defect nor zinc blend phase.

    The formation of the one-sided saw structures may be owing to two factors. First, it could be due to the self-catalyzed process of the Cd-terminated (0001) surface resulting in the asymmetric growth morphology. Secondly, the growth of the side nanoteeth may also be related to the presence of the zinc-blend layer. From the structural information provided by Figure 1(a, c), the top and bottom surfaces of the Wurtzite phase are the ±(2-1-10) low energy facets, but the corresponding planes for zinc blende phase are ±(110), which are the high energy facets for cubic system and are thus energetically unfavorable. Therefore, the width of the zinc blende strip is limited for reducing the surface energy, but a continuous growth as driven by the catalytic active Cd-terminated (0001) surface tends to re-nucleate the Wurtzite phase. This nucleation is epitaxial but a multiple nuclei case along the length of the nanoribbon is most likely, possibly resulting in the growth of the nanoteeth on one side.

    One has to be cautious in identifying phase transformation because electron beam can induce phase transformation as well. Taking ZnS as an example, the wurtzite structured ZnS is unstable under the electron beam illumination in TEM and it may transform to zinc blend structure. The orientation relationship between the two phases are: [2-1-10] || [01-1], and (0001) || (111). The two phases co-exist by sharing the same (0001) or (111) plane. It is also known that the cubic phase ZnS typically has the {111} twins.