New Insights into Planar Defects in Layered alpha-MoO3 Crystals

The observation of regular (h0l) planar defects in alpha-MoO3 crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies because of thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of alpha-MoO3. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to HxMoO3 precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into alpha-MoO3 belt crystals by heating in liquid sulfur at 300 degrees C. These defects are undetectable by both atomic force microscopy and scanning electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of alpha-MoO3 at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges and the argon-plasma-induced Raman spectral evolutions of the as-grown alpha-MoO3 belt crystals provide new insights into the planar defects with regard to their formation and characteristics.