Rapidly counting atomic planes of ultra-thin MoSe₂ nanosheets (1 ≤ n ≤ 4) on SiO₂/Si substrate

The optical, thermal and electrical properties of ultra-thin two-dimensional (2D) crystal materials are highly related to their thickness. Therefore, identifying the atomic planes of few-layer crystal materials rapidly is crucial to fundamental study. Here, a simple technique was demonstrated based on optical contrast for counting atomic planes (n) of few-layer MoSe₂ on SiO₂/Si substrates. It is found that the optical contrast of single-layer MoSe₂ depends on light wavelength and thickness of SiO₂ on Si substrate. The data calculated based on a Fresnel law-based model as well as atomic force microscopy (AFM) measurements fit well with the values measured by spectroscopic ellipsometer. Furthermore, the calculated and measured contrasts were integral and plotted, which can be used to determine the MoSe₂ atomic planes (1 ≤ n ≤ 4) accurately and rapidly.

The optical contrast of thin-layer (1 ≤ n ≤ 4) MoSe₂ sample was calculated by Fresnel’s law, and the contrast was measured by OM image. By combining these two group data, a function of layer numbers and contrast was developed: C (contrast) = 0.07N + 0.177. This proves that contrast is linear to layer numbers of MoSe₂ when layer number is less than 4.