Fingerprints Of Sp(1) Hybridized C In The Near-Edge X-Ray Absorption Spectra Of Surface-Grown Materials

Carbon structures comprising sp(1) chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp(1)/sp(2) character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp(1)/sp(2) materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp(2) systems. We then consider a mixed polymer alternating sp(1) C-4 units with sp(2) biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp(1) hybridization in surface-grown C structures.