Thickness identification of atomically thin InSe nanoflakes on SiO₂/Si substrates by optical contrast analysis

Single layers of chalcogenide semiconductors have demonstrated to exhibit tunable properties that can be exploited for new field-effect transistors and photonic devices. Among these semiconductors, indium selenide (InSe) is attractive for applications due to its direct bandgap in the near infrared, controllable p- and n-type doping and high chemical stability. For its fundamental study and the development of practical applications, rapid and accurate identification methods of atomically thin nanosheets are essential. Here, we employ a transfer matrix approach to numerically calculate the optical contrast between thin InSe flakes and commonly used SiO₂/Si substrates, which nicely reproduces experimental values extracted from optical images on 285 nm SiO₂/Si substrates. Standard 90 and ∼300 nm SiO₂/Si substrates result to provide an optimized optical contrast to detect few-layer InSe flakes using monochromatic illumination at ∼450 and ∼520 nm, respectively. On the other hand, calculated optical contrast reveals an optimum value of 110 nm for the thickness of the SiO₂ film on Si substrates in order to detect InSe nanoflakes as thin as one single layer, under white light illumination. These results demonstrate that the proposed optical contrast method is a very fast and reliable technique to identify atomically thin InSe nanoflakes deposited onto SiO₂/Si substrates.