High-accuracy measurement of the crystalline orientation of anisotropic two-dimensional materials using photothermal detection

Abstract

Anisotropic two-dimensional materials, such as black phosphorus (BP) and rhenium disulfide (ReS₂), possess intrinsically in-plane anisotropic properties that can be tuned for advanced applications, like in thin-film polarizers or polarization-sensitive photodetectors. The crystal orientations should be accurately determined before these materials are applied. However, current methods for measuring the crystalline orientation, such as reflection and transmission spectra, and polarized Raman-related techniques, suffer from significant uncertainty and unreliability because of complex interference, the sample thickness and background signals, which can be particularly strong on transparent substrates. Herein, for the first time, we present a new high-accuracy and background-free method, photothermal detection (PTD), for the identification of the crystalline orientation of BP and ReS₂ on a transparent substrate. PTD is a refractive index sensing optical technique, in which a modulated pump beam is absorbed by BP or ReS₂, leading to a local change in the refractive index of the photothermal medium. The propagation of the probe beam at different wavelengths is modified by the produced periodical change of the refractive index. PTD could be utilized here to identify the crystalline orientation explicitly and accurately. Taking into account the signal generated by the anisotropic optical absorption of the pump beam and the resulting heating of the liquid medium, PTD has no background interference regardless of the excitation wavelength and samples thickness. By analyzing the relationship between the polarization angle of the pump beam and the strength of the photothermal signal, the crystalline orientation can be accurately distinguished. Therefore, PTD overcomes typical challenges associated with transparent substrates, including insulating and rough surfaces, and enables the unambiguous identification of crystalline orientation.