Raman Spectroscopic Fingerprint of Exfoliated S=1 Triangular Lattice Antiferromagnet NiGa₂S₄

Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Now, since the emergence of 2D magnets, there has been even more potential for interesting properties to arise and result in applications ranging from spintronics to topological magnonics. Here we report the first successful exfoliation of bilayer and few-layer flakes of S=1 triangular lattice antiferromagnet NiGa₂S₄. We establish the reported number of layers of the material by performing atomic force microscopy (AFM) and detail a careful characterization using Raman spectroscopy to see how the optical, electronic, and structural properties of the crystal change as a function of sample thickness. Raman active phonons in the material shown to have a strong dependence on excitation wavelength due to resonance effects. Infrared (IR) active modes that had become Raman active due to disorder in the material are shown to have some dependence on sample thickness, however, their behavior with exfoliation has yet to be explained. These results provide a reliable method of determining the number of layers of the material by means of Raman spectroscopy.