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