Fig. 2: Probing interface by adjusting the probing angle or tuning the photo energy.

a Depth profiling by angle-resolved X-ray photoemission spectroscopy. b The inelastic mean free path (IMFP, λ) of photoelectron with different kinetic energy ranges indicated. (The IMFP plots are generated using formulas 5 and parameters 1 reported by Seah and Dench³², namely, \(\lambda = \frac{{177}}{{E^2}} + 0.054\sqrt E\) for “Gold”, \(\lambda = \frac{{143}}{{E^2}} + 0.054\sqrt E\) for “Elements”, \(\lambda = \frac{{641}}{{E^2}} + 0.096\sqrt E\) for “Inorganic Compounds”, and \(\lambda = \frac{{31}}{{E^2}} + 0.087\sqrt E\) for “Organic Compounds”. The units of λ and E are nm and eV, respectively.)