The rediscovery of graphene sparked the research zest on 2D materials. In general, 2D materials can be isolated from their 3D bulk crystals that host a layered crystal structure where the atoms in the same layer are bonded by covalent bonds and the adjacent layers are coupled by van der Waals (vdW) coupling. So far, more than 50 types of 2D materials have been achieved and studied. These materials belong to distinct crystal families and exhibit fabulous physical properties. Among them, 2D telluride materials are of great interests. Here, we present out recent work on Raman spectroscopy on two tullerides: WTe₂ and ZrSiTe. Raman spectroscopy reveals the crystal structure information through the detection of lattice vibration modes. Due to its nondestructive nature and high test efficiency, Raman spectroscopy has been widely used in2D material research on crystal structure and optical characteristics.

In this report, we first present the Raman spectra evolution with the thickness in 2D WTe₂.¹ WTe₂ belongs to transition-metal dichalcogenide (TMD) family. It shows ultra-high non-saturating magnetoresistance up 45 Tesla, leading to great research interest all around the world in recent years. However, when layered TMDs are scaled down to a 2D geometry, structural and electronic transitions occur, leading to the emergence of properties not usually found in the bulk. Here, we report a systematic Raman study of the exfoliated semi-metallic WTe₂ flakes with thickness ranging from few layers down to a single layer. A dramatic change in the Raman spectra occurs between the monolayer and few-layer WTe₂ as a vibrational mode centered at ~86.9 cm⁻¹ in the monolayer splits into two active modes at 82.9 and 89.6 cm⁻¹ in the bilayer. Davydov splitting of these two modes is found in the bilayer, as further evidenced by polarized Raman measurements. Strong angular dependence of Raman modes on the WTe₂ film thickness reflects that the directional interlayer interaction rather, than the isotropic van der Waals coupling is playing an essential role in the phonon modes, especially in anisotropic 2D WTe₂ material. Therefore, the significant evolution of Raman modes with thickness and polarization can not only be a reliable fingerprint of the thickness and the crystallographic orientation, but also an ideal probe for the strong and directional interlayer interaction.

Then, we extend our work to ZrSiTe, a topological nodal line semimetal. It has attracted lots of research interest recently because of the prediction that the single-layered ZrSiTe film is 2D topological insulator. So it is essentially important to investigate the crystalline structure transition occurring in single and few layers of ZrSiTe. Here, we demonstrate our recent Raman study on the lattice vibrational modes of 2D ZrSiTe. Unlike TMDs, ZrSiTe was not considered as a 2D material traditionally because the layers are not bonded by van der Waals coupling. However, due to the weak interaction between adjacent Te layers, ZrSiTe crystal is in the first time separated into the single layers by mechanical exfoliation. In a typical 2D ZrSiTe thin film, 3 Raman modes located at 95 cm^-1, 228 cm^-1, and 304 cm^-1, can be clearly identified, which is in good agreement with theoretical calculations. However, in the single-layered ZrSiTe the intensity of Raman mode located at 131.7 cm^-1 which can be attributed to the mode related to the lattice defects dominates the whole spectrum. This mode fades quickly with the increasing thickness and disappears at 3L. Our experimental and theoretical results manifest that such defect Raman mode can be enhanced by 4.8 times with higher defect concentration which provides a sensitive approach to detect the lattice defects when ZrSiTe reaches to its 2D limit.

In conclusion, Raman spectroscopy is a powerful tool for the investigation of 2D materials. It not only provides an efficient way to determine the thickness of the materials but also shows clues of the interactions inside the crystal. Its sensitivity to the crystal defects makes it a valuable tool to reveal the crystal quality of 2D materials. The use of Raman spectroscopy in 2D material research will finally help us to interpret the physical properties when the materials are thinned down to their 2D geometry.

¹ Yan Cao, Natalya Sheremetyeva, Liangbo Liang, Hui Yuan, Tingting Zhong, Vincent Meunier, and Minghu Pan, 2D Materials 3 (4), 035024 (2017).