Simulation of XPS C1s spectra of organic monolayers by quantum chemical methods.

Several simple methods are presented and evaluated to simulate the X-ray photoelectron spectra (XPS) of organic monolayers and polymeric layers by density functional theory (DFT) and second-order Møller-Plesset theory (MP2) in combination with a series of basis sets. The simulated carbon (C1s) XPS spectra as obtained via B3LYP/6-311G(d,p) or M11/6-311G(d,p) calculations are in good agreement (average mean error <0.3 eV) with the experimental spectra, and good estimates of C1s spectra can be obtained via E(C1s)(exp) = 0.9698EC1s(theory) + 20.34 (in eV) (B3LYP/6-311G(d,p)). As a result, the simulated C1s XPS spectra can elucidate the binding energies of the different carbon species within an organic layer and, in this way, greatly aid the assignment of complicated C1s XPS spectra. The paper gives a wide range of examples, including haloalkanes, esters, (thio-)ethers, leaving groups, clickable functionalities, and bioactive moieties.