Two-dimensional materials as inorganic analogues of graphene: methods of preparation, optical properties, and applications

Abstract

Two-dimensional (2D) materials such as GaSe, InSe, and WSe2 in form of bulk crystal and few-layer sheets have been prepared by mechanical exfoliation method. Atomic force microscopy (AFM) analysis was used for the study of surface morphology and estimation of thicknesses of the exfoliated thin semiconductor layers. Optical absorption and magneto-optical Faraday rotation spectra of 2D materials at different temperatures have been studied. In spectral region near long-wavelength absorption edge for all the studied layered crystals at low temperatures the exciton series have been observed. It was shown that in GaSe and InSe crystals these series consist of three lines, whereas in WSe2 two exciton lines were revealed. The observed exciton series were interpreted in framework of 3D exciton model. In the absorption spectra of GaSe and InSe crystals at higher photon energies than band gap Eg additional exciton structure also was observed. In case of thin exfoliated WSe2 flakes with thickness less than 80 nm shift of ground exciton band to shorter wavelengths has been revealed, which is associated with quantum size effect. Faraday rotation spectra of the 2D crystals have been confirme INTRODd dominant role of excitons near absorption edge.