Mutual Stabilization of Metastable Phases of Tin Oxide: Epitaxial Encapsulation of Tetragonal SnO Microcrystals by Orthorhombic SnO2

Electron microscopy studies on stannous oxide (α-SnO), a metastable two-dimensional layered material, have revealed exciting aspects of the structure and phase stability in the tin–oxygen system. Single-crystalline SnO sheets with an “inverse-pyramid” morphology (thickness of ∼30 nm and lateral dimension in the micron scale) have been synthesized using a simple wet chemical technique. Electron diffraction from the sheets reveals a secondary metastable phase, namely, o-SnO2, to be present coherently with the single-crystalline SnO. The orientation relationship between the two phases and their variants has been investigated. Atomic-resolution imaging reveals the presence of the coherent secondary o-SnO2 phase and its two rotational variants on the surface. A detailed cross-sectional analysis of the sheets reveals that the SnO phase is completely encapsulated by a thin layer of the metastable orthorhombic phase. The metastable SnO phase is thus stabilized by the presence of the encapsulating o-SnO2 on its surface; the metastable o-SnO2 phase itself is stabilized by the underlying SnO substrate. The temperature dependence of resistance, I–V characteristics, and the Schottky barrier height of the lithographically formed SnO/Cr–Au contact on the single crystals have been studied. The transport mechanism is found to be governed by the variable-range hopping (VRH) process. The present study clarifies several unexplained features of the reported microstructures in the SnO system and provides some new insights into the mutual stabilization of metastable phases in the oxides of Sn that could be applicable for other systems as well.