Fig. 2: Charge-contrast enhanced XPS analysis of engineered sapphire surface changes upon air aging.

a Schematic of the XPS setup equipped with an electron flood gun, which emits low-energy electrons to control the surface charging. b Illustration of surface potential modulation as a function of incident electron flux: (i)net positive charging ( + U), (ii) near-neutralization, and (iii) overcompensation leading to net negative charging (-U). c Corresponding energy level alignments at the sample-spectrometer (SP) interface during under different charging conditions. hν is the energy of the incident radiation. e^- means emitted electrons. \({V}_{{{{\rm{s}}}}}\) is the surface potential. \(\varphi\) is the work function. \({E}_{{{{\rm{B}}}},{{{\rm{FL}}}}}\) is the binding energy referenced to the Femi level (FL). \(\Delta {{{\rm{B}}}}\) and \(\Delta {{{{\rm{B}}}}}^{{\prime} }\) are the differential peak shifts between Al-O @ Al₂O₃ and Al-OH @AlO_x(OH)_y without and with surface charge differential contrast (\(\Delta {{{\rm{V}}}}\)). At no or low electron flux (i), uncompensated photoemission results in a net positive surface potential and core-level shifts toward higher binding energies. At moderate flux (ii), partial neutralization occurs, and excess electrons accumulate in surficial Al₂O₃ and AlO_x(OH)_y domains. This induces localized differential charging, enhancing both the O1s and Al2p peak separations between the two surface phases. At high flux (iii), excess electrons create a generally less contrasting negative surface potential. d Relationship between flood gun filament current and resulting electron flux, correlated with C 1 s peak positions used for charge referencing. e Al 2p and O 1s spectra of engineered sapphire after 2 weeks of air aging, acquired under different electron flood gun settings. The charge-contrast effect amplifies the binding energy differences between two surface phases, with the Al-OH bonding energy reduced by 1.5 eV, thereby enabling unambiguous phase differentiation via increased peak separation in both O1s and Al2p spectra.