## 3 Results and Discussion
The different growth modes of SnSe\({}_{2}\) on two substrates may be attributed to the larger local density of states of \(\sqrt{3}\)-Sn/Si(111) than that of graphene substrate, leading to an increased interface adsorption ability [27; 28]. The height of ML SnSe\({}_{2}\) is \(\sim\)0.62 A [Figure 1j], which is consistent with the previous report [23]. Having determined the lattice structure, we then investigated the chemical states of the SnSe\({}_{2}\) film via in situ XPS. The full XPS spectra of Si(111), \(\sqrt{3}\)-Sn/Si(111) and SnSe\({}_{2}\) are shown in the Supplementary Materials Figure S2. Figure 2a presents the Si 2\(p\) core-level spectra of the Si(111)-(7 \(\times\) 7) surface, \(\sqrt{3}\)-Sn/Si(111) and ML SnSe\({}_{2}\) film. For the Si(111)-(7 \(\times\) 7) surface, the core levels of Si 2_p_\({}_{1/2}\) and Si 2_p_\({}_{3/2}\) orbitals are located at 99.7 eV and 99.1 eV, in accordance with the previous literature [29]. The peaks red-shift by about 0.1 eV compared to the core levels of Si 2_p_\({}_{1/2}\) (99.6 eV) and Si 2_p_\({}_{3/2}\) (99.0 eV) orbitals of the \(\sqrt{3}\)-Sn/Si(111), indicating a charge transfer effect from Sn to Si upon the surface Sn-Si bond formation, which is reasonable since the electronegativity of Si is stronger than that of Sn. In addition, after the growth of ML SnSe\({}_{2}\), the core levels of Si 2_p_\({}_{1/2}\) (99.8 eV) and Si 2_p_\({}_{3/2}\) (99.2 eV) again shift toward higher binding energies, which suggests interfacial charge transfer from the \(\sqrt{3}\)-Sn/Si(111) substrate to ML SnSe\({}_{2}\). Subsequently, in Figure 2b, the core levels of Sn 3_d_\({}_{3/2}\) (493.3 eV) and 3_d_\({}_{5/2}\) (484.9 eV) orbitals of \(\sqrt{3}\)-Sn/Si(111) exhibit obvious blue-shift compared to the core levels of Sn 3_d_\({}_{3/2}\) (494.6 eV) and 3_d_\({}_{5/2}\) (486.2 eV) orbitals of ML SnSe\({}_{2}\). This shift can be explained by the much stronger bond polarity of Sn-Se than that of Sn-Si, which leads to the weakened screening to the Coulomb potential created by the positive core of Sn, and finally resulting in the  
Figure 2: (**a**) XPS spectra around Si 2\(p\) orbitals of the Si(111)-(7 \(\times\) 7) surface, \(\sqrt{3}\)-Sn/Si(111), and ML SnSe\({}_{2}\). (**b**) XPS spectra around Sn 3\(d\) orbitals of the \(\sqrt{3}\)-Sn/Si(111) and ML SnSe\({}_{2}\). (**c**) XPS spectra around Se 3\(d\) orbitals of the ML SnSe\({}_{2}\). All the dotted curves are the raw data of the XPS spectra. The red curves are the multiple Lorentzian fitting curves, where the green curves indicate the Shirley background parts, the dark blue and wather curves corresponding to the Si 2_p_\({}_{3/2}\) and Si 2_p_\({}_{1/2}\) fitting peaks in (**a**), Sn 3_d_\({}_{5/2}\) and Sn 3_d_\({}_{3/2}\) fitting peaks in (**b**), Se\({}_{1}\) 3\(d\) and Se\({}_{2}\) 3\(d\) fitting peaks in (**c**), respectively. higher binding energies of the remaining electrons. Moreover, in Figure 2c the core levels of Se 3\(d\) orbitals of ML SnSe\({}_{2}\) split into two sets, Se\({}_{1}\) 3\(d_{3/2}\) (54.6 eV) and Se\({}_{1}\) 3\(d_{5/2}\) (53.7 eV), and Se\({}_{2}\) 3\(d_{3/2}\) (55.0 eV) and Se\({}_{2}\) 3\(d_{5/2}\) (54.2 eV), respectively. The splitting of Se 3\(d\) orbitals implies the presence of two nonequivalent chemical environments of Se atoms in ML SnSe\({}_{2}\). This can be attributed to the fact that the bottom layer of Se atoms is adjacent to the \(\surd\)3-Sn surface, while the top layer of Se atoms is located at the top surface of SnSe\({}_{2}\). To further investigate the interface effect, we characterized the electronic structures of the SnSe\({}_{2}\)/\(\sqrt{3}\)-Sn/Si(111) system via in situ ARPES. Figure 3 shows the electronic structures of the Si(111)-(\(7\times 7\)) and \(\sqrt{3}\)-Sn/Si(111) surface.