# Raman Spectra of ZrS\({}_{2}\) and ZrSe\({}_{2}\) from Bulk to Atomically Thin Layers
## 4 Discussion
Raman signal has been detected from bulk down to three layers for ZrS\({}_{2}\) and five layers for ZrSe\({}_{2}\) on a Si/SiO\({}_{2}\) substrate and down to two layers for ZrS\({}_{2}\) on a silicon substrate covered with Ta (3 nm) and Au (30 nm). Indeed, the Au substrate coating masks the Si peak that appears at 300 cm\({}^{-1}\) and that overlaps with A\({}_{\text{lg}}\) peak of ZrS\({}_{2}\), thus allowing a clearer detection of the ZrS\({}_{2}\) mode. In order to optimize the observed Raman signal for each material, Raman spectra were measured at three different wavelengths (532 nm, 638 nm, and 785 nm) on a thick flake and shown in Figure C6. From the three wavelengths inspected, the optimum ones are found to be 532 nm for ZrS\({}_{2}\) and 638 nm for ZrSe\({}_{2}\). Both materials have an indirect band gap. In the case of ZrS\({}_{2}\), the direct band gap is around 2 eV (620 nm), while the indirect one is at 1.4 eV (885 nm). On the other hand, ZrSe\({}_{2}\) has a direct gap around 1.2 eV (1030 nm) and an indirect gap of 0.7 eV (1770 nm) [35]. Thus, we can ascribe the signal enhancement at these wavelengths to two factors: a resonance effect and an increase of the absorption (more volume contributing to the scattering). However, the thickness of the SiO\({}_{2}\) layer may also play a role, as observed in the Interference Enhanced Raman Scattering (IERS) [24]. In addition, no major changes are observed in the Raman spectra of freshly cleaved and uncleaved bulk samples (Figure C7). However, partial oxidation of the surface cannot be neglected since it is already stated that, with time, bulk materials are subjected to a surface oxidation at room temperature and in ambient conditions [37]. The amount of relative surface oxidation as well as the influence of the substrate is expected to be larger in thin layers than in bulk samples since the ratio between the surface and the volume is higher. Moreover, it should be noted that, especially for thin flakes, special care had to be taken regarding the power density of the laser used during measurements and the acquisition time. Indeed, even using a low laser power density (170 \(\upmu\)W/\(\upmu\)m\({}^{2}\)), flakes were revealed to be easily degraded during the measurement if the exposition time was not sufficiently short. This effect was observed to be more dramatic for ZrSe\({}_{2}\) (on the order of seconds) than ZrS\({}_{2}\) (on the order of minutes). This tendency is also reported and coincides with the one observed in bulk crystals of ZrX\({}_{2}\)[38]. A more extended discussion on flakes degradation and oxidation is reported in Appendix E.  
Finally, we also found that no Raman signal could be obtained at all from non-freshly exfoliated ZrSe\({}_{2}\) thin layers. This could be attributed to the possible oxidation or degradation of the flakes in air, as it has been observed in HfS\({}_{2}\)[39] or NbSe\({}_{2}\)[40]. Despite the fact that ZrSe\({}_{2}\) appeared to be more sensitive to ambient conditions than ZrS\({}_{2}\), we cannot exclude the possibility that monolayers and thin layers of both materials may be unstable in air. In order to avoid this phenomenon, further research should be performed in encapsulated samples or under inert gas conditions [40].