# Raman Spectra of ZrS\({}_{2}\) and ZrSe\({}_{2}\) from Bulk to Atomically Thin Layers
## Appendix B Thin Layers of ZrX\({}_{2}\) Obtained by Mechanical Exfoliation  
Optical images and AFM measurements in layers with different thicknesses of mechanically exfoliated ZrX\({}_{2}\) are shown in Figure 11 for ZrS\({}_{2}\) and in Figure 11 for ZrSe\({}_{2}\).  
Figure 11 reports the optical image and AFM measurement of a thin flake of ZrS\({}_{2}\). From the AFM profile it is possible to estimate the thickness of the thinner part of the flake to be 1.76 \(\pm\) 0.10 nm (profile 1) and the thickness of the thicker part to be 2.32 \(\pm\) 0.11 nm (profile 2). One has to note that thickness estimation has been done on the fitting of the average of five different contiguous profiles. As already reported in the literature, a \"dead layer\" is often observed in mechanically exfoliated flakes due to the ever-present layer of absorbed water, which remains captured between the flake and the substrate [31]. From Figure 11 we found the \"dead layer\" to be about 5 A and from Figure 11 we find a similar value (about 5.9 A), thus allowing us to estimate the previous flakes to be two and three layers thick, respectively. However, it must be noted that, due to the difficulty in the estimation of the \"dead layer\", the thickness of the thicker part of the flake is much smaller than the thickness of the thicker part of the flake.  
Figure 11: XRPD experimental pattern (black) and corresponding fit (peaks in blue and background in green) for ZrX\({}_{2}\) crystals. For simplicity, the fit has been marked with red lines. (**a**) Pattern of ZrS\({}_{2}\). Inset: Photography of a ZrS\({}_{2}\) crystal. The fit gives as a result: a = b = 3.6628(5) Å and c = 5.8270(10) Å, trigonal crystal system with P-3m1 space group, \(\chi^{2}\) = 3.8\(\cdot\)10\({}^{-6}\), and a [PERSON]’s figure of merit of 41.64. (**b**) Pattern of ZrSe\({}_{2}\). Inset: Photography of a ZrSe\({}_{2}\) crystal. The fit gives as a result: a = b = 3.7722(4) Å and c = 6.1310(9) Å, trigonal crystal system with P-3m1 space group, \(\chi^{2}\) = 2.4 \(\times\) 10\({}^{-6}\), and a [PERSON]’s figure of merit of 50.16.  
layer\" and its value comparable to the thickness of one layer of the material, an error of \(\pm\)1 layer could be found in the layers estimation. While the thickness of the first layer is always critical due to the adsorbates presence, the step between the two layers (\(\approx\)0.56 nm \(\pm\) 0.15 nm) is found to be well in agreement with the \(c\) parameter of the unit cell expected for this material (5.827 A).  
Figure 10 reports the optical image and AFM measurement of thin flakes of ZrSe\({}_{2}\). From the first AFM profile we can estimate the thickness of the thin layer to be 0.90 \(\pm\) 0.11 nm. This is compatible with a monolayer and a \"dead layer\" of about 3 A. Flakes measured in profile 2 are thicker and they measure, from right to left: 3.5 \(\pm\) 0.2 nm, 4.8 \(\pm\) 0.3 nm, 9.1 \(\pm\) 0.3 nm and 12.0 \(\pm\) 0.2 nm. Considering the \"dead layer\" of 3 A and the \(c\) parameter of the unit cell of 6.1310 A, they would correspond to five, seven, 14, and 19 ZrSe\({}_{2}\) layers, respectively. Again, in the estimation of the number of layers of these thicker flakes an error of \(\pm\)1 layers could occur. Nevertheless, the small value of the \"dead layer\" found in this case should limit this eventuality.