Science News：2021 Adv. Sci. Overview Optical Inspection of 2D Materials! Promotion of Next-Generation Material-Based Devices
Advanced Science (IF 16.808) published the research by Hsuen-Li Chen et al. from National Taiwan University in October 2021. Optical inspection is a fast and non-destructive method to characterize the properties of two-dimensional (2D) materials. With the advantage of real-time and large-scale monitoring of optical inspection, the industry’s goal of large-scale inspection of the characteristics of 2D materials is not far away, and further acceleration of the mass production of 2D materials and the development of related production equipment has made great progress. This study reviewed several of the important structural properties of 2D materials, including graphene, transition metal dichalcogenides (TMDCs, such as: MoS2), hexagonal boron nitride (h-BN), group III monochalcogenides, black phosphorus (BP) and group-IV monochalcogenides, and discuss how to accurately detect them using appropriate optical inspection techniques.
The author summarizes the main characteristics of these optical inspection technologies (see Table 1). According to their fundamental functions, they can be categorized into four categories.
First, optical spectroscopies (OSs) , spectroscopic ellipsometry (SE) , FTIR spectroscopy, and X-ray spectroscopy (XRS) including X-ray scattering/diffraction/ reflectivity are techniques that can be used to obtain broadband or omnidirectional optical spectra from various 2D materials.
Second, Raman and PL spectroscopies are powerful techniques that can provide rich information about the fine structural characteristics of 2D materials.
Third, s-SNOM, nano-FTIR and AFM-IR spectroscopic methods are based on AFM technology, so they can achieve the highest spatial resolution.
Fourth, single-photon emission, TRPL and pump-probe spectroscopies (PPSs) provide time-dependent optical characteristics for characterizing of 2D materials; information about carrier dynamics can be obtained, including carrier-photon, carrier and carrier-phonon interactions.
The “check mark (✓)” in Table 1 indicates that such structural properties or characteristics can be inspected and easily quantified; “triangular mark (Δ)” means that these characteristics will affect the signal obtained from optical inspection techniques, but their quantifications are not possible or still need further research; “blank cell” means that there has been no previous reports on the use of optical inspection techniques to detect such properties in 2D materials.
Table1. Characteristics of optical inspection techniques for probing 2D materials.
In addition, the authors describe the challenges and opportunities faced by the application of optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques that can be used to enhance the optical signal from 2D materials massively and accurately. It is hoped that through a comprehensive review, greater progress can be made in the development of novel 2D materials-based devices.
Schematic representation of the most widely studied 2D materials.
Doping-related optical properties of 2D materials. (g) When the PL spectra of p-type doped MoS2 is more and more dominated by exciton emission (X0) , it has higher radiative quantum efficiency and higher emission energy, and with the increase of p-type doping concentration, the spectral weight of the trion emission (IX− /Itotal) decreases upon increasing the concentration of p-type doping.