## 3 Results and Discussion
The dotted curves are the raw data, and the red curves are the Lorentzian fittings to the EDCs. (**i–k**) Zoom-in ARPES spectra near the CBM of (**i**) ML, (**j**) FL and (**k**) Multi-L SnSe\({}_{2}\) around the M point, respectively. charge transfer from the \(\sqrt{3}\)-Sn/Si(111) substrate to the grown SnSe\({}_{2}\) film. In addition, the CBM of different layers of SnSe\({}_{2}\) are obtained from energy distribution curve (EDC) fittings, as shown in Figure S3. It can be clearly seen that all the CBM features appear near the Fermi level at the M point, shifting from \(-\)199 meV in ML SnSe\({}_{2}\) to \(-\)112 meV in few-layer (FL) SnSe\({}_{2}\) (the thickness of FL SnSe\({}_{2}\) is estimated to be \(\sim\)3 ML), and finally to the edge of the Fermi level in multi-layer (Multi-L, the estimated thickness is about \(\sim\)6 ML) SnSe\({}_{2}\) [Figure 4i-k]. The upward shift in CBM with the increase in layers again confirms the interfacial charge transfer scenario. Furthermore, the ARPES spectra of ML, FL and Mult-L SnSe\({}_{2}\) along the \(\Gamma\)-M direction are plotted in Figure 4b-d, respectively. The VBM are all located at the \(\Gamma\) point. For more detailed observation, the zoom-in ARPES spectra near the VBM around the \(\Gamma\) point and corresponding EDCs along the red lines are illustrated in Figure 4e-h. Intriguingly, we note that a new band \(\alpha\) above the original VBM of bulk SnSe\({}_{2}\) emerges in the ML and FL SnSe\({}_{2}\) films, which disappears in the Mult-L SnSe\({}_{2}\). The emerging \(\alpha\) band can be better resolved in the second-derivative spectra, as shown in Figure S4. From the EDC fittings in Figure 4h, the maximum of the \(\alpha\) band is located at \(-\)1.05 eV and \(-\)0.97 eV in ML and FL SnSe\({}_{2}\), respectively, leading to the reduced band gaps as 0.85 eV in both of them. The position, as well as the relatively flat dispersion of the \(\alpha\) band, is quite similar to the top valence band of H-phase SnSe\({}_{2}\) (H-SnSe\({}_{2}\)), which was characterized and marked as '7/8' band in a previous study [35]. Moreover, a recent first-principle calculation showed a reduced band gap of H-SnSe\({}_{2}\) compared to T-SnSe\({}_{2}\), in accordance with our ARPES results [36]. Therefore, we believe that the newly emerged energy band \(\alpha\) would be attributed to the presence of a small amount of H-SnSe\({}_{2}\) formed in ML and FL SnSe\({}_{2}\). Indeed, a pressure-induced structural phase transition to H-SnSe\({}_{2}\) has been reported before [37]. Therefore, we assume that the interfacial strain from \(\sqrt{3}\)-Sn/Si(111) substrate would be the main reason for the emergence of H-SnSe\({}_{2}\), which vanishes gradually as the film becomes thicker. Similar interfacial-induced structural phase transitions in other 2D materials were also observed in previous studies. For example, the lattice structure of WSe\({}_{2}\) grown on SrTiO\({}_{3}\) substrate was 1T'-phase [38], while the mixture of 1T'- and 1H-phase WSe\({}_{2}\) was found on graphene [39; 40]. However, this interfacial strain effect is hard to be directly detected by RHEED due to the large relative error (\(\sim\)1.3%) in RHEED analysis, while the lattice constants of H-SnSe\({}_{2}\) and T-SnSe\({}_{2}\) are quite similar. Furthermore, the indistinct band structures of ML and FL SnSe\({}_{2}\), contrary to the clear band structures in Multi-L SnSe\({}_{2}\), also provide evidence for our assumption, since the band structures of both T-SnSe\({}_{2}\) and H-SnSe\({}_{2}\) can be detected by ARPES and eventually overlap together in the spectra, as shown in Figure 4b,c. Our results indicate the potential application prospects of SnSe\({}_{2}\)/\(\sqrt{3}\)-Sn/Si(111) in 2D devices. First of all, the Si is a traditional semiconductor with mature large-scale device fabrication processes, which is beneficial for the practical application of SnSe\({}_{2}\) grown on Si substrate.