Thickness identification of thin InSe by optical microscopy methods

Indium selenide (InSe), as a novel van der Waals-layered semiconductor, has attracted large research interest, thanks to its excellent optical and electrical properties in the ultrathin limit. Herein, four different optical methods to quantitatively identify the thickness of thin InSe flakes on various substrates, such as SiO2/Si or transparent polymeric substrates, are discussed. In the case of thin InSe deposited on a transparent substrate, the transmittance of the flake in the blue region of the visible spectrum is used to estimate the thickness. For InSe supported by SiO2/Si, the thickness of the flakes is estimated by either assessing their apparent colors or accurately analyzed using a Fresnel law-based fitting model of the optical contrast spectra. Finally, the thickness dependency of the InSe photoluminescence emission energy is also studied, which provides an additional tool to estimate the InSe thickness, and it works both for InSe deposited on SiO2/Si and on a transparent polymeric substrate.