Interaction strength and molecular orientation of a single layer of pentacene in organic-metal interface and organic-organic heterostructure

A single layer (SL) of pentacene molecules deposited on the Cu(119) surface and on an organic self-assembled monolayer (SAM) has been investigated by near-edge x-ray absorption fine structure (NEXAFS) at the C $K$ edge, and by photoemission at the $\mathrm{C}1s$ core level. The lowest unoccupied molecular orbitals (LUMOs) of the pentacene SL on the SAM are basically unaffected with respect to that of pentacene in the gas phase, indicating a weak interaction of pentacene with the SAM, while a strong redistribution of the LUMO-related final states is observed when the molecules are deposited on the Cu(119) substrate, sign of an electronic mixing between molecular and metal electronic states, in agreement with recent theoretical predictions. The strong dichroic response of the NEXAFS signal indicates upstanding pentacene molecules oriented 16° off normal for the organic-organic heterostructure. The rehybridization of pentacene orbitals with Cu states at the pentacene/Cu interface hinders an accurate determination of the molecule orientation, which is, however, compatible with molecules lying flat on the Cu(119) terraces.

Figure 1

(a) C $K$-edge NEXAFS spectrum for pentacene (Pn) in the gas phase (from Ref. 29); (b) NEXAFS spectra for a pentacene SL deposited on the Bt-SAM/Cu(001) surface, taken with the electric field vector parallel (gray) and perpendicular (black) to the surface normal. (c) NEXAFS spectra for a pentacene SL on Cu(119), taken with the electric field vector parallel (gray) and perpendicular (black) to the (001)-oriented terraces normal of the Cu(119) surface. The main resonanaces in the ${\pi }^{*}$ and ${\sigma }^{*}$ regions in the NEXAFS spectrum of pentacene and/or SAM heterostructure are marked by dashed lines. All the spectra were recorded for several orientations of the electric field vector by using different angles of incidence of the synchrotron radiation according to the experimental geometry (see text).

Figure 2

(a) XPS $\mathrm{C}1s$ core-level spectrum for pentacene in the gas phase (Ref. 29); the ionization energy scale has been aligned to the binding energy of adsorbed pentacene, shifting the data by $5.0\mathrm{eV}$ toward lower binding energy (Ref. 36); (b) XPS $\mathrm{C}1s$ core-level spectrum at the pentacene SL deposited on the Bt-SAM/Cu(001); (c) XPS $\mathrm{C}1s$ core-level spectrum at the pentacene SL deposited on the Cu(119) substrate. The latter two spectra were collected with a photon energy of $400\mathrm{eV}$.

Figure 3

Left panels: C $K$-edge NEXAFS spectra (circles) collected at different polar angles in the ${\pi }^{*}$ region along with the results of the fitting (solid lines), for pentacene-SL/Bt-SAM (a), and for pentacene-SL/Cu(119) (b); spectra collected for different polar angles $\left(\theta \right)$ between incident photon polarization and substrate surface normal, ranging from 10° (light gray spectrum) up to 90° (black spectrum); the absorption step edge is subtracted from the rough data. Right panels: integral intensity for the components related to transition to LUMO orbitals (triangles) and to $\mathrm{LUMO}+1$ orbitals (filled circles), and the total intensity (open circles), as a function of the polar angle for pentacene-SL/Bt-SAM (solid lines are the results of the fitting procedure) (c), and for pentacene-SL/Cu(119) (the solid lines are a guide to the eyes) (d).

Figure 4

Schematic representation of the coordinates used for our analysis: (a) pentacene/Bt-SAM and (b) pentacene/Cu(119). The ${\pi }^{*}$ orbital involved in the transition is represented by a vector $\mathbf{T}$, with angle $\alpha$ with respect to the surface normal ($z$ axes) and azimuthal angle $\varphi$ with respect to the surface plane. The vector $\mathbf{E}$ represents the linearly polarized electric field vector of the radiation $h\nu$. The radiation falls onto the sample surface plane $\left(xy\right)$ with a fixed grazing angle $\eta$. The polar angle $\theta$ is between the electric field vector and the normal to the (001) surface, while $\delta$ indicates the angle between the vectors $\mathbf{E}$ and $\mathbf{T}$.

Figure 5

Evolution of the integrated intensities of peaks 2 and 3 of the absorption to the LUMO resonance, for the pentacene SL-Cu(119) (the solid lines are a guide to the eyes).