Growth oscillation of MoSe₂ monolayers observed by differential reflectance spectroscopy

The precisely controlled growth of transition metal dichalcogenide (TMDC) monolayers requires sensitive and nondestructive techniques to monitor the morphology and coverage in situ and in real time. In the current work, differential reflectance spectroscopy (DRS) was applied to monitor the molecular beam epitaxy (MBE) growth of atomically thin MoSe₂ layers on mica. The optical evolution exhibits an oscillation with monolayer periodicity, revealing a two-dimensional (2D) layer-by-layer growth of the MoSe₂ thin films. The observed sensitivity of DRS to the step density is associated to the modified electronic structures at the edges of TMDC monolayers. As DRS works in any transparent ambient, we speculate that it could be of great use for realizing precisely controlled growth of TMDC monolayers using not only MBE but also chemical vapor deposition (CVD).