UPS and XPS measurements

In situ STM, XPS, and UPS measurements were performed at room temperature after each annealing treatment to record the evolution of morphology, chemical bonding, and work function, respectively. XPS spectra were recorded using Mg K_α radiation (hv = 1253.6 eV) with a pass energy of 30 eV and an emission angle of 53° with respect to the sample plane. All spectra were calibrated using the reference spectra of a Pt foil. Peak positions for Fe were determined by fitting with Gaussian–Lorentzian functions after subtracting a Shirley nonlinear sigmoid-type baseline, while those for Se were evaluated by the peak maxima. For UPS measurements, both He I and He II light sources (hv = 21.22 and 40.81 eV) were used with an emission angle of 61° with respect to the sample plane. The Fe foil used as a reference was sputtered and annealed in UHV for several cycles to remove contamination and oxides. The peak position for Fe 2p _3/2 was at 706.56 eV. A Se film was obtained by depositing Se on STO substrates at room temperature for 1.5 h, which resulted in a nominal film thickness of 20 nm. The peak maximum of Se 3d was at 55.35 eV. Error bars of the peak positions were estimated by considering the energy resolution of the measurement (0.05 eV).