Fingerprints of sp1 Hybridized C in the Near-Edge X-ray Absorption Spectra of Surface-Grown Materials

Abstract

Carbon structures comprising sp1 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 sp1/sp2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp1/sp2 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 sp2 systems. We then consider a mixed polymer alternating sp1 C4 units with sp2 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 sp1 hybridization in surface-grown C structures.

Scanning tunneling microscopy and Raman spectroscopy of polymeric sp–sp2 carbon atomic wires synthesized on the Au(111) surface

“…Notably, we have verified that the theoretical STM image is not sensitive to the orientation of the molecule on the surface: the simulation of the metalated polymer oriented along the close packed direction, as in ref. 45, is indeed qualitatively very similar to that reported in Fig. 2(e).…”

“…1(b)), the simulations are performed for a model with the wires aligned along the [112] surface direction, as seen in experiment. Note that this geometry differs from previous calculations, 28,45 where the polymer was aligned along, or at a small angle from the [110] close-packed direction. The unambiguous identification of the wire orientation has been possible through the large scale images in which the herringbone Au(111) reconstruction can be easily identified.…”

“…This constraints applies an overall 6.7% strain to the polymer compared to its unit length relaxed in vacuum, similar to the previous studies. 45 The distance between adjacent chains in the orthogonal in-plane direction ([110]) is taken equal to 8.65 Å. The adopted periodicity is slightly smaller than that determined from the experiments but, due to the negligible inter-molecule interaction, it gives reliable results by avoiding the computational effort to include further substrate atoms in the simulation.…”

Structural, Electronic, and Vibrational Properties of 2D Graphdiyne-Like Carbon Nanonetwork Synthesized on Au(111): Implications for the Engineering of sp-sp2 Carbon Nanostructures

“…Noticeably, p x states are sharper in the PDOS than p z states, due to a smaller hybridization with the Au(111) orbitals. Similar observations could be drawn for a linear spsp 2 system 46. Conversely, p y states are sit farther from the Fermi level both in the occupied (valence band) and unoccupied (conduction band) part, as expected from their σ character and direction along the chain axis.Upon removal of the Au adatom, the 2D nanonetwork is decoupled from the substrate and maintains the energy gap around the Fermi level, although reduced with respect to the freestanding nanonetwork (≈ 1.6 eV versus ≈ 2.1 eV).…”

Structural, Electronic, and Vibrational Properties of a Two-Dimensional Graphdiyne-like Carbon Nanonetwork Synthesized on Au(111): Implications for the Engineering of sp-sp2 Carbon Nanostructures

“…Similar observations could be drawn for a linear sp-sp 2 system. 46 Conversely, p y states sit farther from the Fermi level both in the occupied (valence band) and unoccupied (conduction band) part, as expected from their σ character and direction along the chain axis.…”