## 3 Results and Discussion
The constant energy mappings near the Fermi level of the Si(111)-(\(7\times 7\)) and \(\sqrt{3}\)-Sn/Si(111) surface clearly display the six-fold symmetry and the relative 30\({}^{\circ}\) rotation with respect to each other [Figure 3b,c], in accordance with the corresponding schematic of the Brillouin zone (BZ) illustrated in Figure 3a. The band structure of Si(111)-(\(7\times 7\)) surface is shown in Figure 3d, with the intensity enhanced zoom-in spectra plotted at the bottom panel of Figure 3e for more detailed observation. Three surface states marked as S\({}_{1}\), S\({}_{2}\) and S\({}_{3}\) can be clearly observed, in agreement with the previous literature [30]. The S\({}_{3}\) is induced by the back bonds of the (\(7\times 7\)) reconstruction, showing the obvious band dispersion with its maximum located at about \(-1.8\) eV. The nearly flat S\({}_{2}\), induced by the rest-atoms, is located at about \(-0.9\) eV, while the S\({}_{1}\), induced by the adatoms of the surface reconstruction, lies near the Fermi level. The band structure of the \(\sqrt{3}\)-Sn/Si(111) surface and its zoom-in spectra with enhanced intensity are plotted in Figure 3f and the top panel of Figure 3e, respectively. The surface states S\({}_{2}\)' is located at \(-1\)\(--2\) eV, and the minimum of surface states S\({}_{1}\)' is located at \(-0.7\) eV. The S\({}_{2}\)' and S\({}_{1}\)' are attributed to the three back bonds with Si and the one remaining dangling bond of each Sn adatom, respectively [31; 32]. Our results indicate the well  
Figure 3: (**a**) Schematic illustration of the 2D BZs of Si(111)-(\(1\times 1\)) (blue solid line) and \(\sqrt{3}\)-Sn/Si(111) (red dashed lines) surfaces. (**b**,**c**) Constant energy mappings taken near the Fermi level of (**b**) the Si(111) and (**c**) \(\sqrt{3}\)-Sn/Si(111) surface, respectively. The blue and red dashed lines indicate the BZs of Si(111) and \(\sqrt{3}\)-Sn/Si(111). (**d**) ARPES spectra of Si(111) along the K’-\(\Gamma\)’-\(\Gamma\)’-\(M\)’ direction. (**e**) Zoom-in ARPES spectra near the Fermi level of \(\sqrt{3}\)-Sn/Si(111) (upper panel) and Si(111) (bottom panel). (**f**) ARPES spectra of \(\sqrt{3}\)-Sn/Si(111) along the \(\Gamma\)”-\(M\)”-\(\Gamma\)”-\(K\)”-\(M\)” direction. processed substrate, on which the SnSe\({}_{2}\) films can be grown with high-quality flatness and crystallization. Finally, we investigated the electronic structures of SnSe\({}_{2}\) films grown on the \(\sqrt{3}\)-Sn/Si(111) surface, as shown in Figure 4. Recent first-principles calculations and an ARPES study of SnSe\({}_{2}\) showed that bulk SnSe\({}_{2}\) possesses an indirect band gap of \(\sim\)1.07 eV, whose conduction band minimum (CBM) and valence band maximum (VBM) are located along the M-L and \(\Gamma\)-M (K) directions, respectively [33; 34]. In contrast, ML SnSe\({}_{2}\) film exhibits an indirect band gap of \(\sim\)1.69 eV, with CBM and VBM located at the M and \(\Gamma\) points, respectively [34]. The increased band gap of ML SnSe\({}_{2}\) is attributed to the quantum confinement of electrons in quasi-2D samples and the reduction of the screening. The CBM of both ML and bulk SnSe\({}_{2}\) are higher than the Fermi level, making it difficult to be characterized directly via ARPES in pristine SnSe\({}_{2}\). Figure 4a is the constant energy mapping taken near the Fermi level of ML SnSe\({}_{2}\), presenting clear six-fold symmetry, with electron pockets centered at the M points of BZ. It is quite similar to that of the potassium(K)-doped SnSe\({}_{2}\) single crystals, suggesting the  
Figure 4: (**a**) Constant energy mapping taken near the Fermi level of ML SnSe\({}_{2}\). The red dashed lines indicate the 2D BZ of SnSe\({}_{2}\). (**b–d**) ARPES spectra of (**b**) ML, (**c**) FL and (**d**) Multi-L SnSe\({}_{2}\) along the \(\Gamma\)-M direction, respectively. (**e–g**) Zoom-in ARPES spectra near the VBM of (**e**) ML, (**f**) FL and (**g**) Multi-L SnSe\({}_{2}\) around the \(\Gamma\) point. (**h**) EDCs along the red lines in (**e–g**).