Periodic oscillation in the reflection and photoluminescence spectra of suspended two-dimensional crystal flakes

Abstract

Suspended two-dimensional (2D) materials have been widely used to improve the device performances in comparison with the case of supported 2D materials. To realize such a purpose, 2D materials are mainly suspended on the holes of substrates, which are usually used to support 2D materials. The holes beneath the 2D materials are usually full of air. The air layer with the thickness identical to the hole depth will affect the spectral features of the reflection and photoluminescence spectra of suspended 2D materials because there exist multiple optical interferences in the air/2D-flakes/air/Si multilayer structures. However, it is not clear that how the spectral features depend on the hole depth. In this paper, the reflection spectra of suspended multilayer graphene and MoS2flakes as well as the photoluminescence spectra of suspended multilayer MoS2flakes are systematically studied. The reflection spectra of suspended multilayer graphene flakes exhibit obvious oscillations, showing the optical characteristic with periodic oscillations in wavenumber. The oscillation period decreases with increasing the hole depth (or the thickness of the air layer), but is independent of the thickness of suspended graphene flakes. This can be well explained by the model based on multiple optical interferences in the air/graphenes/air/Si multilayer structures, which have been successfully utilized to understand the Raman intensity of ultrathin 2D flakes and substrate beneath the ultrathin 2D flakes dependent on the thickness of 2D flakes, the thickness of SiO2 layer, the laser wavelength and the numerical aperture of objective. The theoretical simulation shows that the oscillation is obviously observable only when the hole depth reaches up to the value on the order of microns. For suspended multilayer MoS2flakes, the reflection and photoluminescence spectra show similar periodic oscillations in wavenumber and the oscillation period is also dependent on the hole depth. The hole depth is measured by the surface profiler. It is found that the calculated oscillation period based on the measured hole depth and multiple optical interference model is usually larger than the experimental one, which is attributed to the existence of the dielectric layer in the holes. The dielectric layer may be the residues after the hole etching process, which have much smaller refractive indexes than substrates and 2D flakes. This results in an increase of the effective hole depth, which becomes larger than the one measured by the surface profiler. The observation of oscillation period in the reflection and photoluminescence spectra of different flakes of 2D materials demonstrates that the periodic oscillation is a general optical characteristic for optical spectra of suspended 2D materials. In principle, the electroluminescence spectra of suspended 2D materials may also exhibit similar periodic oscillations in wavenumber. These findings may be helpful for understanding the optical spectra of various suspended 2D materials and monitoring the existence of the residues in the holes of substrate after the etching process.