In situ growth of ZnO/SnO₂(ZnO:Sn)_m binary/superlattice heterojunction nanowire arrays

Abstract

This work presents a novel growth design of an in situ epitaxially grown SnO2(ZnO:Sn)m superlattice segment on top of ZnO nanowires.

TEM and DFT study of basal-plane inversion boundaries in SnO2-doped ZnO

“…Without the use of proper analytical methodology and failing to study the previous work on Sn-rich IBs, Cao et al [55] concluded that IBs in their SnO 2 (ZnO:Sn) m nanowires are fully occupied by Sn. Similar conclusions were drawn a year later by Park et al [56], followed by three further studies [57][58][59], where Tan et al [59] nicely shows an intensity undulation in HAADF-STEM image indicating that the IB-layer is probably not fully occupied by Sn. On the other hand, it is great to read the studies by Eichhorn et al [60] and Hoemke et al [63] that have built on this knowledge and constructed fascinating modulated structures with parallel Sn-rich basal-plane IBs making use of either Ga 2 O 3 or Al 2 O 3 additions to stabilize the transient tail-to-tail configuration inbetween, respectively.…”

“…Similar conclusions were made by other authors [53,54]. Observation of IBs in Sn-doped ZnO nanobelts and nanowires was followed by highly competitive studies of their atomic structure using scanning transmission electron microscopy (STEM) methods [55][56][57][58][59]. While most of these studies correctly interpret the translation state of Sn-rich IBs, their lack in understanding of the local charge balance [15] is reflected in erroneous interpretation of the occupancy and arrangement of Sn atoms in the IB-plane.…”

“…Park et al [56] reported the d IB(Zn-Zn) distance of 6.02 Å for the SnO 2 -doped ZnO nanobelts, which is similar to that of the Sb-doped nanorods [77]. Other STEM images in their work vary from 5.55-6.45 Å, with their DFT optimized model predicting a d IB(Zn-Zn) distance of 6.35 Å. Jiang et al [58] determined a similar value in the SnO 2 (ZnO:Sn) m polytypoidic nanowires. The d IB(Zn-Zn) value reported by Eichhorn et al [60] is considerably shorter, but we must keep in mind that these IBs are not directly comparable as they constitute a homologous [Sn 0.5 Zn 0.5 ]GaO 3 (ZnO) 3 phase.…”

“…When studying Sn-rich IBs at high resolution, we realize that all important contrast features are averaged out in crystal foils thicker than ~10 nm. This problem also appears to be the main obstacle in solving the cation distribution in Sn-rich IBs by other authors [55][56][57][58][59], where only the latest work shows some contrast variations along the IB-plane in [011 -0] projection. Why this appears to be such a problem?…”

“…While it is generally known that Sn alone cannot produce a stable tail-to-tail inversion [10] further deficiency in these studies is seen in their knowledge on stabilization of parallel Snrich IBs. Not being aware of contamination with Ga [55], In [57], Si [58] or Al [59] in their experiments, they report on growing pure polytypoidic SnO 2 (ZnO:Sn) m nanowires, instead of fully characterizing these fascinating mixed homologous phases with potentially interesting physical properties. Namely, in addition to Fe and Ti, these dopants are known to stabilize tail-to-tail configuration of IBs [60][61][62][63][64][65], enabling growth of mixed homologous structures with Sn-rich IBs, as this was clearly demonstrated by Eichhorn et al [60] and Hoemke et al [63] for gallium and aluminum additions.…”