Raman Spectroscopy of Few‐Layers TaS₂ and Mo‐Doped TaS₂ with Enhanced Superconductivity

Abstract

are strongly-bonded in the a-b plane but weakly-bonded and stacked by van der Waals forces in the c-direction, which gives them their low-dimensional character. [3][4][5][6][7] The weak van-der-Waals stacking of the planes allows the isolation of single-or few-atomic-layers-thick samples through relatively simple mechanical exfoliation techniques. [8,9] One of the most outstanding characteristics of this family of materials is that, depending on chemical composition, their electronic properties can range from band insulators to metals, and they can show exotic quantum phenomena such as charge density waves (CDWs), magnetic ordering, superconductivity, topological electronic states, among others. [4,10] Among all the members of this family of compounds, the semiconducting members are one the most studied materials in the last 2 decades. Their visible or near-visible range bandgaps, elevated electronic mobilities, large spin-orbit interaction, large susceptibility to changes in physical parameters, spatial scalability, among other characteristics, makes them ideal candidates for a large number of optoelectronics, spintronics, and electronics applications. [3,4,[11][12][13] Interestingly, all of these characteristics get enhanced or only appear in the limit of few atomic layers, and the body of literature characterizing electronic and physical properties in the monolayer limit is extensive. [14,15] The use of simple, fast, and economic experimental tools to characterize low-dimensional materials is an important step in the process of democratizing their use. Raman spectroscopy has arisen as a way of indirectly determining the thickness of nanolayers of transition metal dichalcogenides (TMDs), avoiding the use of more expensive tools such as atomic force microscopy, and it is therefore a widely used technique in the study of semiconducting TMDs. However, the study of many metallic TMDs in the limit of few atomic layers is still behind when compared to their semiconducting counterparts, partly due to the lack of similar alternative characterization studies. In this work the characterization of the Raman spectrum, specifically of the E 2 2g g 1 1 -and A 1g -modes, of mechanically exfoliated Ta 1−x Mo x S 2 , a metallic TMD which exhibits charge density wave (CDW) formation and superconductivity, is presented. The clear identification of contributions coming from the SiO 2 / Si substrate allowed the isolation of the individual E 2 2g g 1 1 -and A 1g -modes of the samples and, for the first time, the observation of a clear evolution of their Raman shifts as a function of sample thickness. This provides a way of indirectly determining sample thickness in the limit of few atomic layers in Ta 1−x Mo x S 2 .

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T′ Transition Metal Dichalcogenide Nanosheets

“…193−196 With increasing number of layers, the A 1g and E 2g vibrational modes of the TaS 2 flakes obviously blueshift and redshift, respectively. A detailed line profile analysis established the relationship between the number of layers and Raman peak positions, 196 which was similarly valid for other TMDs, such as 1T′ MoTe 2 and 2H MoS 2 . 195,197 Second, structural identification and anisotropic orientation could be studied using polarized Raman spectroscopy.…”

“…First, the thickness could be identified based on Raman shifts, which is an alternative option for determining the number of layers. − With increasing number of layers, the A 1g and E 2g vibrational modes of the TaS 2 flakes obviously blueshift and redshift, respectively. A detailed line profile analysis established the relationship between the number of layers and Raman peak positions, which was similarly valid for other TMDs, such as 1T′ MoTe 2 and 2H MoS 2 . , Second, structural identification and anisotropic orientation could be studied using polarized Raman spectroscopy. ,− High built-in strain caused by the surface roughness or wrinkles induces the splitting of the shear phonon mode in WS 2 on the Au/W substrate. The intensity of the split E′ phonons shows both orthogonality and strong dependence on the polarization angle .…”