Identifying the crystal orientation of epitaxially grown MoO2 nanoflakes on c-sapphire

Identifying the crystallographic orientations and understanding the growth mechanism of MoO 2 nanocrystals are important for device applications. Here, we investigate the effect of precursor concentrations in chemical vapor deposition on the orientation of MoO 2 nanoflakes with respect to the substrate surface. X-ray diffraction, optical microscopy, (tilted) scanning electron microscopy, high resolution transmission electron microscopy, selected area electron diffraction, and angle-resolved polarized Raman spectroscopy were used to determine the crystal orientations of the nanoflakes and their epitaxial relationship with c- sapphire substrates: lying-down MoO 2 nanoflakes for MoO 2 (0   1   0)∥sapphire(0001) and MoO 2  <0   0   1>//sapphire< 1 1 ¯ 00 >, and standing-up ones for MoO 2 (1   0   0)∥sapphire(0001) and MoO 2  <0   0   1>∥sapphire< 1 1 ¯ 00 >. Tentative atomic epitaxial models are proposed to explain the orientation and alignment. The investigations also demonstrated that angle-resolved polarized Raman spectroscopy is a quick and nondestructive method to identify the crystallographic orientations of nanoflakes.