Hunting for Monolayer Oxide Nanosheets and Their Architectures

In two-dimensional materials, thickness identification with a sufficient characterization range is essential to fundamental study and practical applications. Here, we report a universal optical method for rapid and reliable identification of single- to quindecuple-layers in oxide nanosheets (Ti0.87O2, Ca2Nb3O10, Ca2NaNb4O13). Because of their wide bandgap nature (Eg = ∼4 eV) and zero opacity, most oxide nanosheets exhibit a weak white-light contrast (<1.5%), which precludes optical identification. Through a systematic study of the optical reflectivity of Ti0.87O2 nanosheets on SiO2/Si substrates, we show that the use of thinner SiO2 (∼100 nm) offers optimum visualization conditions with a contrast of >5%; the contrast is a nonmonotonic function of wavelength and changes its sign at ≈550 nm; the nanosheets are brighter than the substrate at short wavelengths and darker at long ones. Such a nonmonotonic optical response is common to semiconducting oxide nanosheets, including Ca2Nb3O10 and Ca2NaNb4O13. The optical contrast differences between the substrates and nanosheets with different numbers of layers were collected, serving as a standard reference from which the number of layers can be determined by optical microscopy. Our method will facilitate the thickness-dependent study of various oxide nanosheets and their architectures, as well as expedite research toward practical applications.