Optical Inspection of 2D Materials: From Mechanical Exfoliation to Wafer‐Scale Growth and Beyond

Abstract

Optical inspection is a rapid and non-destructive method for characterizing the properties of two-dimensional (2D) materials. With the aid of optical inspection, in situ and scalable monitoring of the properties of 2D materials can be implemented industrially to advance the development and progress of 2D material-based devices toward mass production. This review discusses the optical inspection techniques that are available to characterize various 2D materials, including graphene, transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), group-III monochalcogenides, black phosphorus (BP), and group-IV monochalcogenides. First, the authors provide an introduction to these 2D materials and the processes commonly used for their fabrication. Then they review several of the important structural properties of 2D materials, and discuss how to characterize them using appropriate optical inspection tools. The authors also describe the challenges and opportunities faced when applying optical inspection to recently developed 2D materials, from mechanically exfoliated to wafer-scale-grown 2D materials. Most importantly, the authors summarize the techniques available for largely and precisely enhancing the optical signals from 2D materials. This comprehensive review of the current status and perspective of future trends for optical inspection of the structural properties of 2D materials will facilitate the development of next-generation 2D material-based devices.

Intrinsic Mechanical Strain in Chemical Vapor Deposition-Grown MoS2/MoSe2 Monolayer Heterostructures: Implications for Nanoelectronic Devices

“…Interestingly, this shift of the Raman peaks is not or barely discussed in works related to strained or vertical heterostructures built from transferred TMDs. ,,, More recent works, however, exhibit a phonon hardening of all Raman modes for MoS 2 and WSe 2 when subjected to high pressures. , In 2016, M. S. Kim et al, when studying dry transferred TMDs, observed some small Raman shifts of the first order bands and related them to electron doping in MoS 2 and hole doping in MoSe 2 resulting from charge transfer. Nevertheless, when MoS 2 is submitted to electron gate doping, one observes that there is a downshift of the out-of-plane A′ 1 mode, together with an increase of its fwhm, while the E′ mode does not change, which is explained because of the weaker electron–phonon coupling of the in-plane mode. − When the gate voltage is tuned to induce a hole doping, there are no observable changes in the Raman modes at all. The same behavior was observed for other mono- and bilayer TMDs, including MoSe 2 .…”

A Researcher’s Perspective on Unconventional Lab-to-Fab for 2D Semiconductor Devices

“…2D materials have been investigated widely using standard optical spectroscopy, spectroscopic ellipsometry, Raman spectroscopy, and photoluminescence spectroscopy. They are also frequently characterized using time-resolved PL, pump–probe spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray scattering, diffraction, and reflectivity . These characterization techniques are used for various purposes: to locate the grown 2D materials, to investigate defects in the structure, to probe strain in the structure, to detect the concentration and nature of dopants in the structure, to study carrier dynamics, to identify polaritons or functional groups in 2D materials, to probe lattice spacings, to observe the van der Waals integration, and so on .…”

“…Additionally, optical anisotropy can be correlated to the intensity ratio of Raman D and G bands in graphene . Although used primarily for graphene, spectroscopic ellipsometry provides more reproducible data with higher resolution than optical spectroscopy by measuring the amplitude ratio of reflected light rather than the absolute intensities of reflected and transmitted light …”

“…They are also frequently characterized using time-resolved PL, pump−probe spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray scattering, diffraction, and reflectivity. 106 These characterization techniques are used for various purposes: to locate the grown 2D materials, to investigate defects in the structure, to probe strain in the structure, to detect the concentration and nature of dopants in the structure, to study carrier dynamics, to identify polaritons or functional groups in 2D materials, to probe lattice spacings, to observe the van der Waals integration, and so on. 106 The small and ultrathin 2D flakes of 2D materials isolated from their bulk crystals by mechanical exfoliation are often difficult to detect using atomic force microscopy (AFM), scanning electron microscopy, or transmission electron microscopy (SEM or TEM) owing to their low throughput.…”

“…106 These characterization techniques are used for various purposes: to locate the grown 2D materials, to investigate defects in the structure, to probe strain in the structure, to detect the concentration and nature of dopants in the structure, to study carrier dynamics, to identify polaritons or functional groups in 2D materials, to probe lattice spacings, to observe the van der Waals integration, and so on. 106 The small and ultrathin 2D flakes of 2D materials isolated from their bulk crystals by mechanical exfoliation are often difficult to detect using atomic force microscopy (AFM), scanning electron microscopy, or transmission electron microscopy (SEM or TEM) owing to their low throughput. Optical microscopy-based techniques defining units like optical contrast C r 107 and total color difference (TCD) 108 have been used to visualize graphene flakes on Si/SiO 2 substrates.…”