Raman spectroscopy of CrSiTe$_{\mathrm{3}}$ multilayers*

Ferromagnetism in two-dimensional (2-D) single crystals is a hypothesis under intense scrutiny by theorists and experimentalists alike seeking to model and realize it, respectively. Derived from an intrinsically ferromagnetic bulk compound composed of weakly coupled layers, 2-D CrSiTe$_{\mathrm{3}}$ is an ideal candidate for such a study. Here we correlate the thicknesses of various CrSiTe$_{\mathrm{3}}$ multilayers to their Raman spectra, accounting for the degradative effects of oxidation by fabricating devices and performing measurements in a chemically inert atmosphere. We then passivate the surfaces via encapsulation with hexagonal boron nitride (hBN) before repeating the experiments in air. Our results explain the absence of predicted phonon modes in the few-layer limit of CrSiTe$_{\mathrm{3}}$ as previously observed in the literature and, more importantly, offer a reliable method of identifying the CrSiTe$_{\mathrm{3}}$ monolayers that may potentially settle the debate over ferromagnetic order in reduced dimensionalities.