Low-Frequency Shear and Layer-Breathing Modes in Raman Scattering of Two-Dimensional Materials

Abstract

Ever since the isolation of single-layer graphene in 2004, two-dimensional layered structures have been among the most extensively studied classes of materials. To date, the pool of two-dimensional materials (2DMs) continues to grow at an accelerated pace and already covers an extensive range of fascinating and technologically relevant properties. An array of experimental techniques have been developed and used to characterize and understand these properties. In particular, Raman spectroscopy has proven to be a key experimental technique, thanks to its capability to identify minute structural and electronic effects in nondestructive measurements. While high-frequency (HF) intralayer Raman modes have been extensively employed for 2DMs, recent experimental and theoretical progress has demonstrated that low-frequency (LF) interlayer Raman modes are more effective at determining layer numbers and stacking configurations and provide a unique opportunity to study interlayer coupling. These advantages are due to 2DMs' unique interlayer vibration patterns where each layer behaves as an almost rigidly moving object with restoring forces corresponding to weak interlayer interactions. Compared to HF Raman modes, the relatively small attention originally devoted to LF Raman modes is largely due to their weaker signal and their proximity to the strong Rayleigh line background, which previously made their detection challenging. Recent progress in Raman spectroscopy with technical and hardware upgrades now makes it possible to probe LF modes with a standard single-stage Raman system and has proven crucial to characterize and understand properties of 2DMs. Here, we present a comprehensive and forward-looking review on the current status of exploiting LF Raman modes of 2DMs from both experimental and theoretical perspectives, revealing the fundamental physics and technological significance of LF Raman modes in advancing the field of 2DMs. We review a broad array of materials, with varying thickness and stacking configurations, discuss the effect of in-plane anisotropy, and present a generalized linear chain model and interlayer bond polarizability model to rationalize the experimental findings. We also discuss the instrumental improvements of Raman spectroscopy to enhance and separate LF Raman signals from the Rayleigh line. Finally, we highlight the opportunities and challenges ahead in this fast-developing field.

Anisotropic Phonon Response of Few‐Layer PdSe2 under Uniaxial Strain

“…Their thicknesses were determined using the LF Raman spectra of interlayer vibration modes (Figure S8C,D, Supporting Information), which has been proved to be an accurate and effective method for fast and unambiguous identification of the number of layers of PdSe 2 and many other 2D materials. [ 39,51 ] The angle‐resolved Raman spectra under 532 nm laser excitation further confirmed the crystallographic orientation of the thick region of flake (Figure S8A, Supporting Information): the mode maximum intensities all appeared at 0 ° , corresponding to the a ‐axis of the PdSe 2 crystal, and the b ‐axis was identified to be along 90 ° . The PdSe 2 flake was then transferred from the SiO 2 /Si substrate to a cross‐shape flexible polyimide substrate for the following strain‐related studies.…”

“…Raman spectroscopy is a powerful characterization tool for studying the physical properties of anisotropic 2D materials. It has been widely used to determine the crystallographic orientations, [ 2,5,27–29 ] layer numbers, [ 30–39 ] and strains of various 2D materials. [ 40–48 ] In particular, the phonon response under strain was found to be anisotropic in several 2D materials such as black phosphorus and ReSe 2 .…”

Raman spectroscopy of graphene-based materials and its applications in related devices

“…1(a). [43][44][45][46][47][48][49][50][51][52][53][54] Raman spectra of all graphene-based materials show few prominent features regardless of the final structure. 55 However, the positions, line shapes, and intensities of these peaks give abundant useful information for the investigation of the structures and electronic properties of graphenebased materials.…”

Microcavity enhancement of low‐frequency Raman scattering from a CsPbI3 thin film

“…Obtained Raman spectra can provide insights into numerous properties, including morphology, stress/strain, crystallinity, doping level, conductivity, local temperature, and polarizability of a bulk, a thin film, or a nanostructure. Applications of Raman spectroscopy can be found in physical sciences, life sciences, drug discovery, and semiconductor technology . As most materials have unique vibrational chemical fingerprints, this technique has major advantages over other material characterization techniques in that it is nondestructive, no sample preparation is needed, and finally, the required sample volume can be as small as few cubic microns, which is elementary for monolayers and single‐molecule detection.…”

“…Raman scattering in the low‐frequency spectral range of 5–200 cm −1 is referred to as low‐frequency Raman (LFR). The vibrations detected in LFR region are generated by harmonic oscillations such as lattice phonons and localized intermolecular interactions such as Van der Waals interactions, π–π stacking, and hydrogen bonds . The LFR signal is sensitive to the vibrational modes associated with the material nanostructure and is therefore being used to characterize chiral purity of organic crystals and formulations, biomolecular assemblies, metal–organic frameworks, semiconductor superlattices, and halide perovskites .…”