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

Identifying the crystallographic orientations and understanding the growth mechanism of MoO2 nanocrystals are important for device applications. Here, we investigate the effect of precursor concentrations in chemical vapor deposition on the orientation of MoO2 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 MoO2 nanoflakes for MoO2(0 1 0)IIsapphire(0001) and MoO2 <00 1>//sapphire<1100>, and standing-up ones for MoO2(1 0 0)IIsapphire(0001) and MoO2 <00 1>IIsapphire<1100>. Tentative atomic epitaxial models are pro-posed 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 orienta-tions of nanoflakes.