R. Heyler, J. Carriere, A. Roy
Keywords: inter-layer shear mode, few-layer graphene, transition metal dichalcogenides (TMD), MoSe2, MoS2, layered 2D materials, low-frequency Raman spectroscopy, terahertz spectroscopy
Summary:Two dimensional (2D) materials have attracted a lot of attention due to their optimal optoelectronic properties which are not found in the domain of silicon based electronic materials. The identification and characterization of 2D materials, especially differentiation of few-layer from the bulk phase, is crucial for progress in materials research and manufacturing process development. Raman spectroscopy, and more specifically low-frequency/THz-Raman® analysis, has been recently introduced as a highly sensitive, real-time and non-destructive analytical tool for these novel materials. THz-Raman® spectroscopy incorporates both Stokes and anti-Stokes low-frequency signals, which cover the ±150GHz-6THz spectral regime and provide a unique “structural fingerprint” in addition to standard “chemical fingerprint” signals, making it an ideal “one-instrument/one-measurement” characterization tool to study 2D material structures and manufacturing processes. Graphene was the first 2D material to be discovered (2004) and possesses exceptional mechanical and electrical properties. The frequencies of interlayer vibrations in these few layer 2D materials occur in the THz frequency regime and manifest themselves as low frequency Raman peaks close to the laser Rayleigh line. THz-Raman® peaks have been used to characterize both the number of layers and also their stacking configurations. The fact that graphene is a conductor sets some limitations on its potential applications. An alternate class of materials to graphene, which have found traction in recent years, are collectively labelled Transition Metal Dichalcogenides (TMD or TMDC). TMDs like MoSe2 and MoS2 have a band gap that can be chemically tuned to have a wide variety of electronic properties. These materials can potentially be used in FET transistors, spin- and valley-tronics, among other applications. Akin to graphene, the interlayer shear modes of MoSe2 can be clearly seen in the low-frequency/ THz-Raman spectrum, with peaks located at ~18cm-1. These signals are indicative of both the number of layers and their orientation as shown in Figures 1 and 2. Other important structural attributes such as folds and twists can also be analyzed with this technique. Figure 3 shows the corresponding shifts due to orientation changes in the18cm-1 peak, going from 18.4cm-1 to 18.9cm-1 +/- 0.1cm-1 and accompanied by a dramatic shift in magnitude. We thank Dr. Alexander Puretzky (ORNL) for providing the MoSe2 samples for this work.