Optical determination of MoSe$_{\mathrm{2}}$ layer number

We mechanically exfoliate mono- and few-layers of molybdenum diselenide. The exact number of layers is determined by atomic force microscopy, high-resolution Raman spectroscopy, and photoluminescence. We have quantitatively summarized the relation between Raman A$_{\mathrm{1g}}$ mode positions and the layer numbers, from both 532nm wavelength and 633nm wavelength Raman lasers. The spectrum analysis is based on 1-4 layer MoSe$_{\mathrm{2}}$ flakes. These observations provide useful information for the future opto-electronic devices based on these materials.