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

Abstract

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.

Keywords: C 1s absorption; carbynes; density functional theory; near edge X-ray absorption spectroscopy; on-surface chemistry; self-assembly.

Figure 1

(a,b) Band structure (left) and corresponding density of states (right) for infinite 1D carbon chains of the cumulene family (=C=C=, BLA=0) and polyyne one (-C ≡ C-, BLA = 0.172 Å) [54], respectively. The DOS is projected over C 2p atomic orbitals, with x along the chain axis and $y,z$ orthogonal to it. (c) Simulated near-edge X-ray absorption fine structure (NEXAFS) spectrum for different photon polarizations. Energies are reported with reference to the average C 1s ionization potential $E_{\mathrm{thr}}^{\mathrm{avg}}$. The wiggles that are especially evident at low energy are an artifact induced by the use of periodically repeated unit cells (here one C atom every 40 is excited).

Figure 2

(a) Structural model of freestanding poly(p-phenylene) with alternating tilted phenyl rings and (b) the corresponding 1D band structure and PDOS. (c) Simulated NEXAFS spectrum for different photon polarizations (axes as in panel (a)).

Figure 3

(a) Structural model of bBEBP/Au(111). (b,c) Electronic band structure projected on the C atoms belonging to the C${}_4$ section and the biphenyl one, respectively. Left and right panels report the calculations on Au(111) and free-standing, respectively. Darker colors indicate stronger localization over the C atoms, whereas white area denote the absence of electronic state projecting over the selected set of orbitals at that energy. The labeled circles mark the energy of highest occupied (H) and lowest unoccupied (L) electronic states, computed for the free-standing polymer, depicted in Figure 4. (d–f) The same band structure projected on the $p_x$, $p_y$, and $p_z$ orbitals of the C${}_4$ part.

Figure 4

Isosurfaces corresponding to a squared-wavefunction amplitude of $0.01$ Å${}^{-3}$, for the highest occupied (HOMO), the lowest unoccupied (LUMO), and a few more empty electronic states of the freestanding bBEBP polymer (in the geometry shown in Figure 3a, but without the Au surface atoms). For comparison to the LUMO + 1, the LUMO computed for C${}_4$H${}_2$ is shown at the bottom.

Figure 5

Simulated NEXAFS spectrum of bBEBP/Au(111). See Figure 3a for the definition of the $x,y,z$ polarization axes.

Figure 6

Decomposition of the NEXAFS spectrum of bBEBP/Au(111) into contributions by the sp${}^1$ chain (C${}_4$) and the sp${}^2$ biphenyl group (BP). (a) Spectrum averaged over all polarizations. (b) In-plane electric field along the polymer axis x, and (c) orthogonal to it, y; (d) out-of-plane z-directed electric field (see Figure 3a for the definition of $x,y,z$ axes).

Figure 7

(a) Structural model of bBEBP/Au(111) embedding a Au adatom (red) in between two organic units. (b) Simulated polarization-averaged NEXAFS spectrum (solid) compared to the one of Figure 6a (without the Au adatom, dashed). (c) Polarization-resolved NEXAFS spectrum; see panel (a) for the definition of the $x,y,z$ polarization axes.

Figure 8

(a) Structural model of 2D network formed by de-bromination of tBEP/Au(111) [30] (here simulated as if it were freestanding). (b) Simulated NEXAFS spectrum for in-plane ($x,y$) and out-of-plane (z) photon polarizations.