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Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface Sabine-Antonia Savu, Giulio Biddau, Lorenzo Pardini, Rafael Bula, Holger Friedrich Bettinger, Claudia Draxl, Thomas Chasse, and Maria Benedetta Casu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03768 • Publication Date (Web): 11 May 2015 Downloaded from http://pubs.acs.org on May 19, 2015

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The Journal of Physical Chemistry

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface Sabine-A. Savu,⊺ Giulio Biddau,‡ Lorenzo Pardini,‡ Rafael Bula,† Holger F. Bettinger, † Claudia Draxl,‡ Thomas Chassé,⊺ M. Benedetta Casu *⊺

⊺

Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der

Morgenstelle 18, D-72076 Tübingen ‡

Physics Department and IRIS Adlershof, Humboldt-Universität zu Berlin, Zum Großen

Windkanal 6, 12489 Berlin Germany †

Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076

Tübingen

Corresponding Author *E-mail: [email protected], Tel. +49 7071 29 76252, Fax: +49 7071 29 5490 (M.B.C.).

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Abstract. Although physisorption is a widely occurring mechanism of bonding at the organic/metal interface, contradictory interpretations of this phenomenon are often reported. Photoemission and X-ray absorption spectroscopy investigations of nanorods of a substituted pentacene, 2,3,9,10-tetrafluoropentacene, deposited on gold single crystals reveal to be fundamental to identify the bonding mechanisms. We find fingerprints of a fractional charge transfer from the clean metal substrate to the physisorbed molecules. This phenomenon is unambiguously recognizable by a non-rigid shift of the core-level main lines while the occupied states at the interface stay mostly unperturbed, and the unoccupied states experience pronounced changes. The experimental results are corroborated by first-principles calculations.

Keywords. organic/metal interface, physisorption, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, density functional theory


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Introduction The key to tuning device performance is the understanding of the various mechanisms that occur at their interfaces. The interfaces form the device, and the type of these interfaces (e.g. metal/metal, metal/semiconductor, or semiconductor junctions) along with related phenomena (Schottky barrier vs. ohmic contacts) define, together with the stability and the properties of the active layers,1-2 the electronic characteristics, the performance, and the lifetime of a device. Although organic/metal interfaces have been the focus of nearly two decades of investigations,3-7 very recently the interest in this type of interface has enjoyed a renaissance with the flourishing of a body of work focused mainly on the interfaces between chemisorbed organic molecules and metals.8-10 Photoemission features strongly depend on the strength of the molecule/substrate interaction, as demonstrated for a number of molecules on Ag(111).9 These experiments show that the stronger the bond of the molecules with the substrate, the larger the effect of the charge transfer on photoemission is.9 Resonance structures are stabilized on the surface through an initial metal-to-molecule charge transfer and rehybridization of suitable side groups, leading to an extended π-electron system that is strongly coupled to the metal states.8 This coupling, decreasing the molecular electronic gap, overcomes the competing phenomenon of Fermi-level pinning, and leads to substantially charged molecular monolayers. If the Fermi level comes to lie within a frontier molecular orbital, the monolayer behaves as metallic. The change from semiconducting to metallic nature of the organic material is suggested as a new route for the chemical engineering of metal surfaces.8 The present “state of the art” in the interpretation of the organic/metal and organic/organic interfaces, is based on the interface interaction strength, considering the complete range of interactions from physisorption of noble gases to strong chemisorption of π−conjugated molecules.11-12 Recently the organic/metal interface has been described within the framework of the interface density of gap: Defect and chemistry-induced gap states may originate at the 3 ACS Paragon Plus Environment


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metal/organic interface because of the formation of chemical bonds and/or defects between the organic semiconductor and the metal, thus, in this case a charge transfer to the gap states may occur.12-13 The case of physisorption with and without charge transfer is also examined within the integer electron charge transfer model, stating that physisorption on organic and passivated metal surfaces is possible, while weak chemisorption, with possible fractional charge transfer, occurs on non-reactive clean metal surfaces.11 These models, although extremely useful and detailed, do not explain all experimental results because physisorption is a widely occurring phenomenon at the organic/metal interface, exhibiting different spectral characteristics that depend on the strength of the molecule/metal interaction. In a first approximation, it is possible to understand the mechanisms of adsorption of a molecule on a solid surface by using the theory of chemical bonds,1 and using simple considerations as derived by mathematical models such as the Lennard-Jones potential.14 However, looking at a continuous curve with no singularities, such as the Lennard-Jones potential, it is easily understood that some variations of forces, in particular their ranges, might impede understanding the bonding mechanisms, i.e. chemisorption versus physisorption. This is especially true when applying the potential to a different dimensionality, as in case of a molecule adsorbed onto a surface. The case of weakly interacting metal-semiconductor interfaces was addressed in the 1960’s with inorganic semiconductors, by using the concept of tunnelling of the wavefunctions from the metal into the semiconductor, 15-17 and suggesting the central role of the metal-semiconductor strength in the charge transfer at interface. We here contribute to the understanding of the organic/metal interface by an experimental and theoretical multi-technique investigation, choosing 2,3,9,10-tetrafluoropentacene18 (F4PEN, Figure 1), a fluorinated pentacene derivative, as a model system. Pentacene-based molecules are potential candidates for organic electronics due to the fact that substitution is a very powerful way to tailor the optical and electronic molecular properties to specific technological 4 ACS Paragon Plus Environment

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needs.19 Indeed, unsubstituted pentacene (PEN), a p-type semiconductor, is the subject of numerous investigations owing to its high charge-carrier mobility and its ability to form highly oriented thin films.3-4, 20-26 These properties can be tuned, according to specific technological needs, for example, by fluorination. This turns pentacene into the n-type semiconductor perfluoropentacene.19, 27-28 We investigate F4PEN molecules deposited on Au(110) single crystals by using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The simultaneous use of this variety of techniques, in combination with controlled in-situ deposition, gives the opportunity not only to explore the complete electronic structure of the systems (occupied and unoccupied states, or orbitals) but to avoid possible artefacts and discrepancies due to slightly different preparations that could impact the morphology and the structure of the assemblies and consequently their electronic structure.29-31 We demonstrate that a fractional charge transfer from the metal substrate to the physisorbed molecules occurs and that it exhibits a very specific and clearly recognizable fingerprint: This is a non-rigid shift of the XPS main lines and at the interface a strong alteration of the NEXAFS signal together with almost unperturbed occupied states. The experimental results are corroborated by calculations based on density-functional theory (DFT), as implemented in the exciting code.32

Experimental methods Sample preparation and photoemission experiments were performed in an ultrahigh vacuum (UHV) system consisting of a preparation and an organic chamber (base pressure better than 10-9 mbar) and an analyzing chamber (base pressure 8·10-10 mbar) equipped with a Specs Phoibos 150 analyzer, a monochromatic Al Kα source, and a high-flux He discharge lamp (UVS 300 Specs) with an excitation energy of 21.22 eV. The secondary electron cut-off was 5 ACS Paragon Plus Environment


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measured with a bias potential of -7V. The sample work function was determined as the energy difference between the photon energy and the width of the whole spectra (secondary electron cut-off to the Fermi edge) taking the applied voltage into account. A clean Au(110) single crystal (Surface Preparation Laboratory) was used as a substrate, prepared by several cycles of sputtering (Ar+ ion bombardment: 1250 V) and annealing (600 K). The cleaning cycles were repeated till XPS showed no traces of contaminants, UPS the appropriate work function, and low energy electron diffraction (LEED) the expected pattern. No deposition has been performed on contaminated substrates, as a standard procedure. The preparation of the nanorods was carried out in-situ under UHV conditions by using organic molecular beam deposition (evaporation rate: 1.3 Å/min, substrate temperature, Tsub =RT). The evaporation rate was measured with a quartz crystal microbalance. Survey XPS spectra were recorded using pass energy of 50 eV, detailed spectra were measured with 20 eV pass energy. The experimental resolution (200 meV) was calculated from the broadening of the Fermi edge. The He I β and He I γ satellites were subtracted from the experimental data for all UPS spectra. NEXAFS measurements were performed at the UE52-PGM undulator beamline at BESSY (Berlin). The measurements were carried out in single bunch (top up mode, ring current = 13.6 mA, cff = 2.5, 40 µm exit slit, analyzer resolution= 0.1 eV). The main chamber (base pressure 2x1010 mbar) was equipped with a standard twin anode X-ray source, and a SCIENTA R4000 electron energy analyzer. We carried out NEXAFS measurements in the partial electron yield mode, in grazing incidence (70° w.r.t. the sample normal). We measured the spectra by using linearly polarized synchrotron radiation parallel (s-pol) and normal (ppol) to the surface, changing the direction of the polarization by means of the undulator. The NEXAFS spectra were normalized by taking the clean substrate signal and the ring current into account. All spectra were scaled to give an equal edge jump. Details on NEXAFS normalization are given elsewhere.22, 33 22, 33 No degradation of the samples was observed on the time scale of all discussed experiments. 6 ACS Paragon Plus Environment

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Calculation Details Calculations were performed within the framework of density-functional theory and using the full-potential augmented planewave plus local orbital (APW+lo) method,34 as implemented in the exciting code.32 With the APW+lo basis, the Kohn-Sham equation of DFT is solved without any shape approximation for the potential, even close to the nuclear cores. This allows for treating all electrons on the same footing and, hence, accessing the core region. Exchange and correlation effects were accounted for by the local-density approximation (LDA).35 For the single-molecule calculations, the F4PEN molecule was placed in an orthorombic unit cell with the x axis parallel to the long molecular axis and the z axis normal to the molecular plane. The amount of vacuum required to prevent interactions between neighbouring molecules is 12 bohr in the x and y directions and 13 bohr in z direction. All calculations were performed relaxing the atomic positions down to a residual force of 5 x 10-5 Ha/Bohr on the individual atoms. The images were obtained within the Tersoff-Hamann approximation based on the calculated local density of states (LDOS).36 For the F4PEN monolayer we used a unit cell defined by in-plane vectors of 33.184 x 13.663 bohr, and a monoclinic angle of 71°. This setup compares fairly well with perfluoropentacene monolayer on graphite.37 In the direction normal to the monolayer, we use the same amount of vacuum as considered for the isolated molecule. The Brillouin-zone was sampled with a Monkhorst-Pack 2x6x1 k-point mesh. Structural relaxation was performed for fixed unit-cell parameters. To allow for additional electronic charge mimicking the metal-to-molecule charge transfer and keeping, at the same time, the overall charge-neutrality of the periodic system, we increased the nuclear charge of carbon atoms by a tiny amount of 0.034. The core-level shifts were obtained as the difference between the respective energy levels of the charged and neutral system using the HOMO as a reference.



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Results and discussion In Figure 1, the thickness-dependent core level spectra of F4PEN nanorod assemblies are shown (for nanorod morphology and XPS stoichiometric analysis after deposition see Supporting Information). The C1s core level spectra of the thicker assemblies are dominated by a peak at 285.5 eV and a further peak at 286.4 eV. The main peak can be assigned to carbon atoms in the ring of the backbone (carbon atoms bound only to carbon (CC) or also to hydrogen (CH)), while further features at higher binding energy are related to carbon atoms which are bonded to fluorine (CF). With increase of the nominal thickness, the C1s main line is shifted toward higher binding energy by ~1 eV. A widely spread satellite structure which is typical for acenes38 is also visible at higher binding energy (see Supporting Information for a zoom into this region). We observe that with increasing thickness the spectroscopic lines do not experience a rigid homogeneous shift: The core level shift is more pronounced for the main line that is 1.04 (main line at lower binding energy, for contributions related to the pentacene backbone) compared to 0.98 eV (for contributions related to CF, see Figure 1a). The thickness-dependent F1s spectra show a single line at 688.0 eV (Figure 1b), as expected because of the presence of fluorine atoms that have the same chemical environment. A 0.85 eV shift toward higher binding energies is visible comparing the thickest and the thinnest assemblies. Note that the line shape and width slightly change with thickness. XPS line shape is influenced by several parameters, (see also the discussion in the computational part below), in case of P4PEN this is related i) to the structural changes30 because of the reorientation effect experienced by the molecules with thickness (see Supporting Information); ii) to the presence of the fluorine atoms that impact the electronic charge redistribution, due to their strong electronegativity39; iii) to the nanorod morphology. 39 Apart from these aspects, no other relevant changes in XPS line shape and intensity are observable. Thus, we conclude that the molecular density of states stays unperturbed at the interface. The shake-up satellite features are also visible in the spectra of the thinnest 8 ACS Paragon Plus Environment

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assemblies at higher binding energies; however, their intensity does not show abrupt changes at the interface (see Supporting Information). These observations hint at physisorption, which is further supported by the fact that the molecules are almost completely desorbed after a short annealing at 415 K (see Supporting Information). Therefore, a possible chemical bond at the F4PEN/Au interface can be ruled out as a reason for the observed non-rigid shift of the spectroscopic lines. Note that also the He I UPS spectra (see Supporting Information) do not show any evidence for chemisorption.7 This observation supports the conclusion that the molecules are physisorbed on the surface. In fact, chemisorption would significantly modify the photoemission features of the F4PEN molecular assemblies close to the interface,8-9 as also discussed for XPS above. We observe only a slight shift (~0.16 eV) of the highest occupied molecular orbital (HOMO) onset 40 towards higher binding energies (see Supporting Information). This observation does not exclude the possibility that the non-rigid shift may originate from a local image-charge at the interface that gives rise to a different screening of the various atoms in the molecule. This aspect can be explored by NEXAFS spectroscopy because image-charge screening at the interface does not affect the intensity of the NEXAFS resonances:33, 41 If only image-charge screening occurs, we do expect no significant changes in the NEXAFS spectra collected for the thin and the thick assemblies. As a reference, we first measure the thickness-dependent NEXAFS spectra of diindenoperylene (DIP) thin films deposited on gold (Figure 2). DIP thin films are extensively investigated and DIP is known to physisorb on gold and to show only image-charge screening effects at the organic/metal interface 30-31, 41-43. DIP molecules are flat-lying on the gold surface and they adopt the herringbone motif in thicker films (see Supporting Information for further details).30-31, 41-43 In DIP NEXAFS spectra the same features, identified by the same photon energy, are present at the interface and in the thick films. The structural effect, i.e., the reorientation of the molecules, affects only the feature intensities that are always visible. Because of the specific geometry of the experiment (see Figure 1e), the matrix elements describing the NEXAFS 9 ACS Paragon Plus Environment


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transition rate within the dipole selection rule approximation are non-zero44-45 also in spectra of flat-lying molecules investigated in in-plane polarization (see Figure 2). On the contrary, the intensity of the F4PEN π*-resonances, which represent the prominent spectral features in the range up to around 288 eV photon energy in the NEXAFS spectra surprisingly shows a strong thickness-dependent difference (Figure 1, the experiment was performed exactly as for DIP; see Supporting Information for F4PEN polarization dependent NEXAFS experiments). These resonances have lower or missing intensity (see energy range between and 283.5 and 284.5eV, as evidenced in Figure 1d). A similar behaviour was found for nanorod assemblies of other substituted pentacenes.39, 46 As discussed for DIP in case of perfectly flat lying molecules, the π*-resonance intensity still contribute and the transition matrix elements are nonzero 44-45 if only structural or charge-image effects are expected (see Figure 2). Therefore, F4PEN NEXAFS behaviour cannot be ascribed to purely structural reasons. A non-rigid shift is reported in the literature for other physisorbed organic molecules like 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA),47 cobalt phtalocyanine,48 and magnesium phthalocyanine.49 This behaviour is irrespective of the specific orientation of the molecules on the substrate that ranges from flat lying (PTCDA47 and phtalocyanine48-49) to recumbent (substituted pentacenes39, 46). The non-rigid shift is also observed irrespective of the character of the substituents (electron-accepting or electron-donating groups), for example, it is also recognisable in 2,3,9,10-tetramethoxy-pentacene assemblies.46 Thus, while photoemission experiments unambiguously demonstrate that the molecules are physisorbed, i.e., there is no evidence of a change in the molecular density of states at the interface, in NEXAFS, we observe a clear change of the π∗-resonances, although the valence band of the molecular assembly is unperturbed.46 We interpret this behaviour as being due to charge transfer from the metal substrate to the molecules. 10 ACS Paragon Plus Environment

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DFT calculations support our interpretation. To mimic the thin assembly that experiences charge transfer from the substrate, we consider a single F4PEN molecule and add different amounts of electronic charge. The sampling depth of our experiment is around 54 Å, corresponding to the estimated inelastic mean free path of 18 Å50. It means that the XPS spectra of the 254 Å nominally thick assembly do not contain contributions from the interface. Consequently, the thick assembly results can be compared to neutral molecules since in this case the charge transfer at the interface is masked. The theoretical approach of considering single molecules is justified by the fact that, as seen by XPS and UPS, the metal/molecule interaction is weak and the molecular orbitals do not change their character upon adsorption5153

as we will discuss below in more detail. The results are presented in Figure 3a, where the

calculated core-level shifts are depicted as a function of transferred charge. A non-rigid shift of the core levels towards lower binding energies is clearly observed. By a direct comparison of the theoretical results with the experimental ones, we can deduce that the experimental values are well reproduced when 0.75 electrons are added to the molecule. In this case, in fact, we find shifts of 1.16 and 1.14 eV (comparable with the experimental shift of 1.04 eV of the XPS main line) for contributions related to CH and CC; 1.01 eV for CF (to be compared with the experimental value of 0.98 eV); and 0.79 eV for F atoms (corresponding to 0.85 eV in the experiment). In other words, all the theoretical values nicely reproduce the experimentally observed core level shifts (see also Figure 1). Our calculations show that in the “charged” system, the increased screening gives rise to diminished nuclear attraction, pulling the core levels upwards. Note that our findings do not change when a core hole is included in the calculations.54 Figures 2b and 2c show the comparison of the XPS core level spectra with a convolution of delta functions representing the computed core states of the charged and neutral molecule, respectively. This convolution is based on a Voigt profile adopting a Lorentzian with a full-width at half maximum (WL) of 0.1 eV and a Gaussian with a fullwidth at half maximum (WG) of 1 eV. The Voigt profile is chosen in order to take into 11 ACS Paragon Plus Environment


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account both the finite core-hole lifetime (which has a Lorentzian profile) and the broadening due to the finite experimental resolution as well as various inhomogeneities, e.g., molecular packing and local morphology30 (Gaussian profile). Since the interpretation of the experimental results in terms of single molecules might appear simplistic, we perform analogous calculations for an F4PEN monolayer of flat-lying molecules for the case of 0.75 electrons charge transfer. In fact, the results for the single molecules are very well reproduced as indicated by the stars in Figure 3a. The maximum deviation, i.e. 0.08 eV, is obtained for CF, while both CC and CH increase by only 0.01 eV, and F1s experiences a slightly larger shift of 0.07 eV. We consider 0.75 electrons as the upper limit of the transferred charge since in this quantification we accumulate also contributions due to other possible sources of (rigid) corelevel shifts: These are changes in the molecular orientation,55 different charge redistribution, due to the strong electronegativity of the fluorine atoms39 when comparing surface and bulk environment of the molecules (surface core level shift50, 56) and image-charge screening effects due to the capability of the substrate to screen the core-hole generated in the photoemission event.7 The experimental finding that no appreciable change in the HOMO takes place upon charge transfer (compare the UPS spectra of the thick and thin assemblies in Supporting Information) is also supported by our calculations. To this extent, Figure 3d shows the calculated molecular orbital images in the Tersoff-Hamann approximation for the HOMO of the neutral and charged molecule, respectively. Concomitant to the UPS experiment, the orbital does not change its shape.

Conclusions From the combination of various experimental probes and first-principles calculations to determine the electronic structure of physisorbed nanorod assemblies of F4PEN, we find 12 ACS Paragon Plus Environment

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evidence for a charge transfer from a non-reactive clean metal surface to the unoccupied states of the physisorbed molecules. This has specific photoemission and X-ray absorption spectral fingerprints. We quantify the observed core-level shifts as being due a fractional charge transfer. Our work, revealing the true nature of the occurring interface phenomenon, contributes with a consistent understanding to the picture of the organic/metal interface previously drawn by other works.8-9, 11 Our results also show that for weakly bound systems, a single molecule approximation can be a reliable approach for the description of the occupied states of a physisorbed molecule, since these orbitals do not change upon adsorption. This is evidenced experimentally and theoretically. A variety of different complex phenomena occur at the metal interface in physisorbed systems. These may involve either charge transfer and/or charge image screening. These mechanisms affect the occupied and the unoccupied states in different ways: Their common characteristic is to leave the occupied states almost unperturbed. To explicitly identify their nature (e.g., charge transfer versus image screening) the investigation of the unoccupied states plays a fundamental role. Finally, what is most important is the question if this approach may have a more general relevance, beyond the present experiments. In this respect, we have analysed a large number of previous published works in the literature (see also Supporting Information for a more schematic view) and we have observed that strong chemisorption has been associated with big spectroscopic differences in the spectroscopic lines in XPS, UPS and NEXAFS when comparing monolayer versus multilayer.9 Weak chemisorption presents at least two different behaviors: Large spectroscopic differences in the XPS, and NEXAFS when comparing monolayer versus multilayer, and UPS almost unaffected or, alternatively, differences in the UPS, and NEXAFS spectra when comparing monolayer versus multilayer, but XPS stays almost unaffected.11 Weak physisorption with local differences in charge image at the interface it is accompanied by a non-rigid shift in XPS lines and it does not show relevant 13 ACS Paragon Plus Environment


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spectroscopic differences in the XPS, UPS and NEXAFS spectra when comparing monolayer versus multilayer.57 On the contrary, weak physisorption with homogenous charge image at the interface shows a rigid shift of the XPS lines and it is characterized by no relevant spectroscopic differences in the XPS, UPS and NEXAFS spectra when comparing monolayer versus multilayer.27, 41 Our work together with the discussed correlation with previous works demonstrates that the combination of thickness-dependent photoemission and X-ray absorption spectroscopy reveals the fingerprints to unambiguously describe the adsorption mechanisms at the metal/organic interface.

Acknowledgements The authors thank the Helmholtz-Zentrum Berlin (HZB), Electron storage ring BESSY II, for providing beamtime, the HZB resident staff for beamtime support, S. Pohl, W. Neu and E. Nadler for technical support. Financial support from the Helmholtz-Zentrum Berlin is gratefully acknowledged. GB, LP, and CD appreciate support from the Austrian Science Fund (Project I543) and the German Research Foundation (DFG, through the Collaborative Research Project 951). MBC acknowledges the support of DFG through the contract CA852/5-1 and CA852/5-2.

Supporting Information A typical atomic force microscopy (AFM) image evidencing the nanorod morphology. Table S1 with the NitPyn stoichiometry and integrated XPS experimental signal intensities for a thin and a thick assembly. The XPS survey spectrum after annealing at 415 K. Zoom into the higher binding energy range of Figure 1a to evidence the shake-up satellite intensities. Expanded energy scale of Figure 1a and Figure 1b to evidence the non-rigid shift. Table S2 with the parameters used for a best fit analysis of F4PEN XPS curves. Polarization dependent 14 ACS Paragon Plus Environment

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DIP NEXAFS spectra for thin and thick films. Thickness dependent UPS spectra. Polarization dependent F4PEN NEXAFS spectra for a thin and a thick assembly. Schema of the fingerprints. This material is available free of charge via the Internet at http://pubs.acs.org

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Page 21 of 24

Figure 1.

1.04 eV

0.85 eV

F1s

Å 4 5 2

Å 4 5 2

C1s

0.98 eV

C- K NEXAFS

NEXAFS Intensity [a. u.]

Intensity [a.u.]

Å 8 1

Å 8 1

Intensity [a.u.]

Å 8 9

Å 8 9

Å 2

Å 2

thick assembly

thin assembly 294 291 288 285 282 Binding energy [eV]

280

693 690 687 684 681 Binding energy [eV]

a)

290 300 310 Photon Energy [eV]

b)

NEXAFS Intensity [a.u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c)

thick thin

hν

substrate 280

d)

320

285 Photon Energy [eV]

290

e)

Figure 1. a) Thickness dependent C1s, and b) F1s core level spectra of F4PEN assemblies. The corelevel shifts toward higher binding energy with increasing thickness are indicated. c) C K-NEXAFS spectra for nominal thicknesses of 20 Å (thin assembly, lower) and 175 Å (thick assembly, upper), measured in in-plane polarization. The arrows evidence the resonances that experience the strongest changes, as discussed in the text. d) Curves as in Figure 1c: Zoom in the π*-region and overlapped curves to evidence their difference. e) Sketch of the NEXAFS experiment. The molecular structure is also shown.

ACS Paragon Plus Environment

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NEXAFS Intensity (a. u)

Figure 2

NEXAFS Intensity (a. u)

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Page 22 of 24

280

285 Photon Energy (eV)

290

280

285

290

Photon Energy (eV)

Figure 2. DIP NEXAFS spectra zoomed in the π*-region obtained from a submonolayer, (left) and 36 Å DIP thick films (right), deposited on Au(100) single crystals at RT. The spectra were taken in grazing incidence for p- (blue curve) and s- (red curve) polarisation. The geometry of the experiment is the same as in the F4PEN case (see Supporting Information for further details).


22

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Figure 3

2.0

Intensity [a. u.]

C 1s

Core level shifts [eV]

CH CC CF F ML

1.5 1.0

DOS XPS

Thick assembly Neutral molecule

Thin assembly Charged cell

0.5 0.0 0.0

0.5 1.0 Charge [e]

1.5

290 288 286 284 282 280 Binding energy [eV]

a)

Intensity [a. u.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b)

F 1s

DOS

Thick assembly Neutral molecule

XPS

Thin assembly Charged molecule

694 692 690 688 686 684 Binding energy [eV]

c)

d)

Figure 3. a) Theoretical core level shifts of carbons and fluorine atoms versus the added amount of charge. Positive values represent shifts towards lower binding energies. Positive values on the x axis mean that a fractional (negative) electron charge is added to the cell. The arrow indicates the amount of charge transfer comparable with the experimental findings. b) XPS C1s and c) XPS F1s core level spectra for the thin (2 Å) and thick (254 Å) assembly, as indicated. The blue curves are convolutions of the computed core levels aligned to experiment with respect to the main line. d) Images for the HOMO orbital of the neutral (left) and charged (right) molecule. They are calculated from the LDOS integrated in the energy range [-1.0, 0.0] (neutral molecule) and [-1.5, 0.0] eV (charged molecule).


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