Layer-dependent optical properties of PtSe2 determined from a single film

In this work, we have used in situ spectroscopic ellipsometry to investigate the thickness-dependent dielectric functions of PtSe2, a transition metal dichalcogenide material that exhibits interesting optical and electrical properties. Continuous ellipsometry measurements were obtained while a ∼25 nm-thick PtSe2 film was grown using molecular beam epitaxy. By using Kramers–Kronig-consistent oscillators to represent the dielectric functions, we fit the ellipsometry spectra to obtain the dielectric function and the thickness of PtSe2 during its entire growth cycle. The dielectric function changes significantly with thickness, where the centroid of three oscillators used to represent the band-to-band transitions of PtSe2 redshifts as the thickness increases, reaching a saturation value as the film approaches ∼20 nm. Furthermore, the Drude contribution also changes with thickness. More significantly, we can decipher the sample composition as a function of time by using an effective medium approximation-model to represent the film.