Multi-layered Mo, prepared using an in-line sputtering system, was selenized by reaction with Se vapor and thermal annealing of bilayer Mo/Se samples. In situ high-temperature X-ray diffraction analysis indicated that the phase evolution of the glass/Mo/Se sample during heat treatment was similar to that of the selenization of glass/ Mo with Se vapor, except for crystallization of Se in the glass/Mo/Se sample. However, the MoSe2 layer formed from the selenization of the Mo layer by Se vapor preferentially grew perpendicularly to the Mo surface, whereas MoSe2 formed from the reaction of Mo with Se liquid showed random orientation. The detailed reaction pathways of the double-layer random-MoSe2/vertical-MoSe2 formation from the Mo/Se bilayer sample were suggested on the basis of the several characterization results including X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and selected-area electron diffraction patterns. (C) 2012 Elsevier B.V. All rights reserved.