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Electronic Structure of Hexacene and Interface Properties on Au(110) Peter Grüninger, Ma#gorzata Polek, Milutin Ivanovic, David Balle, Reimer Karstens, Peter Nagel, Michael Merz, Stefan Schuppler, Ruslan Ovsyannikov, Holger F. Bettinger, Heiko Peisert, and Thomas Chassé J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04274 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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The Journal of Physical Chemistry

Electronic Structure of Hexacene and Interface Properties on Au(110)

Peter Grüninger,†,‡ Małgorzata Polek,† Milutin Ivanović,† David Balle,+ Reimer Karstens,+ Peter Nagel,§ Michael Merz,§ Stefan Schuppler,§ Ruslan Ovsyannikov,# Holger F. Bettinger,‡ Heiko Peisert†* and Thomas Chassé†

†

Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18,

72076 Tübingen, Germany ‡

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

Germany §

Karlsruher Institut für Technologie, Institut für Festkörperphysik, 76021 Karlsruhe, Germany

#

Institute for Methods and Instrumentation in Synchrotron Radiation Research, Helmholtz-Zentrum

Berlin für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT Although hexacene was first synthesized in 1939 the thin film properties, which are interesting for future applications and fundamental research, were never investigated. Therefore, we synthesized hexacene by reduction of 6,15-hexacenequinone and evaporated films of variable thickness on Au(110). This allowed to study the electronic properties and molecular orientations in the bulk as well as at the molecule-metal interface by X-ray absorption and photoelectron spectroscopy (XAS, PES). Valence band spectra of a multilayer hexacene film are compared to electronic states obtained from DFT calculations. C 1s core level spectra show typical satellite structures of the extended aromatic π-system, similar to pentacene. XAS shows that anisotropy rises with decreasing film thickness and indicate that hexacene is almost flat lying on the Au(110) substrate. The different peak shape of XAS spectra as a function of the film thickness, as well as changes in valence band spectra and C 1s satellite structures indicate a strong electronic coupling of the molecular states with the states of the Au(110) substrate at the interface.


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INTRODUCTION Pentacene is the largest well characterized member of the homologous series of the acenes, and today considered a prototypical organic semiconductor in a number of applications including thin film transistors and organic photovoltaics.1-6 Due to smaller HOMO-LUMO gaps and smaller reorganization energies, the larger acenes were considered as semiconductors with superior properties.7-9 However, the stability of the higher homologues decreases rapidly with the number of annulated rings and consequently little is known about molecular and interface properties of larger acenes.10-13 Although the next higher homologue of the acene series, hexacene, was first synthesized already in 193914 and a number of improved syntheses were reported,9, 15-16 the surface science of this molecule was not investigated for decades. Only in 2017 the adsorption geometry and electronic structure of hexacene monolayers generated on-surface after deposition of a suitable precursor onto Au(111) was studied by scanning tunneling microscopy and spectroscopy.17 During the last year submonolayers of acenes like heptacene, nonacene, and decacene were similarly generated by on-surface synthesis.18-20 However the growth and properties of molecular thin films prepared by physical vapor deposition of acenes larger than pentacene were not investigated so far, presumably due to the limited access and their high reactivity. Our recent observation is that even heptacene has sufficient stability in the solid state to characterize it by solid state NMR spectroscopy.21 The characterization of the film properties of acenes larger than pentacene is deemed essential for possible future applications of these acenes in devices such as organic thin film transistors. Here, we introduce a new approach for the synthesis of hexacene and study interface properties and the film formation on Au(110) by physical vapor deposition. While on-surface preparations typically result in low coverages, our approach allows the growth of hexacene films with variable thickness and enables the characterization of the films by state of the art surface science techniques. The Au(110) substrate was chosen as example for a metal substrate with weak chemical reactivity. Furthermore, on both Au(110)22-24 and Cu(110) metal surfaces the related pentacene molecule forms highly ordered films.



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EXPERIMENTAL SECTION Synthesis of hexacene. For the present study we prepared hexacene (Scheme 1) by a modified Meerwein-Ponndorf-Verley

reduction25

of

hexacene-6,15-quinone

(see

Supporting

Information for detailed synthesis) similar to the procedures described earlier in the Ph.D. theses of Fang and Einholz.26-27 Scheme 1. Synthesis of hexacene by modified Meerwein-Ponndorf-Verley (MPV) reduction of hexacene-6,15-quinone.

While these authors reported the isolation of dihexacenes, we obtained hexacene as blue solid after high vacuum gradient sublimation in an overall isolated yield of only 5%. This procedure allows separation of hexacene and dihydrohexacenes that sublime at lower temperature. The analytical data of the blue solid are in agreement with hexacene and previously reported spectral properties.21 In particular, no indication for the presence of oxidation products was found. Hexacene is stable enough to handle it at room temperature in a glove box where it was transferred into a Knudsen cell for high vacuum sublimation onto an Au(110) single crystal. Methods. The Au(110) single crystal was cleaned by cycles of argon ion sputtering and annealing. The sputtering was carried out at a voltage of 800 V for typically 30 min, subsequently the annealing was performed for 30 min at a temperature of 850 K. Hexacene was evaporated at rates of about 1-4 Å/min determined by a quartz microbalance. All values of the film thickness were obtained by the comparison of photoemission intensities of substrate and overlayer related peaks assuming layer-by-layer growth. Atomic cross sections were taken from Ref.

28.

According to the crystal structure of the related pentacene, the

molecule-molecule distance in vapor grown crystals is about 0.35 nm.29 Assuming flat lying molecules, the thickness of a (nominal) monolayer (ML) was estimated to be 0.35 nm.


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Synchrotron radiation based X-ray absorption spectroscopy (XAS) and photoemission (PES) measurements have been performed at the WERA beamline of KARA (former ANKA, Karlsruhe, Germany) and at PM4 (Helmholtz-Zentrum Berlin für Materialien und Energie) using the Low-Dose endstation equipped with an conventional photoelectron analyzer and an ARTOF analyzer used for angle dependent measurements.30 The absorption was monitored indirectly by measuring the total electron yield (sample current), the energy resolution was set to about 100 meV at a photon energy of 285 eV. For polarization dependent measurements, the polar angle was varied while keeping the azimuthal orientation of the ppolarized light fixed in [1-10] direction of the Au(110) single crystal. The XAS spectra have been normalized to the same step height well above the ionization threshold. The energy resolution of PES spectra was 720 meV, 375 meV, 250 meV and 65 meV at excitation energies of 1000 eV, 500 eV, 385 eV and 140 eV, respectively. Measurements in the home lab were performed using a multi-chamber UHV system (base pressure of 2 × 10−10 mbar) equipped with a Phoibos 150 Hemispherical Energy Analyzer (SPECS), X-ray source with monochromator (XR 50 M SPECS), Ultraviolet Source (UVS 300 SPECS) and Omicron LEED system. Excitation energies were 1486.74 eV and 21.22 eV for XPS and UPS, respectively. All photoemission spectra were taken at normal emission, the angle between analyzer and photon source was 45°-54°, depending on the spectrometer. Peak fitting of XPS spectra was performed using the program Unifit 2018.31 For DFT calculations geometry optimizations in D2h were performed using the hybrid density functional B3LYP32-33 with the 6-31G* basis set as implemented in Gaussian 09.34



RESULTS AND DISCUSSIONS Electronic Structure in the Bulk. We studied the bulk electronic structure of hexacene (HEX) in thin films grown on Au(110) using photoemission and XAS. First, the occupied electronic structure will be discussed. In Figure 1a a typical C 1s core level spectra is shown for a HEX multilayer film grown on Au(110). The slightly asymmetric peak shape indicates the presence of different carbon components and may be compared to C 1s of pentacene (PEN) (see Supporting Information, Figure S2). The film thickness is about 6 nm in both cases, thus bulk properties of the molecules are probed independent of the substrate surface. The C 1s peak maximum for HEX is found at a binding energy (BE) of 284.50 eV, while the BE of the C 1s peak maximum for PEN is observed at a slightly lower energy 284.25 eV (see Supporting Information, Figure S2). The latter is comparable to PEN on Au(111),35 but significantly lower (~1.0 eV) than PEN on Ag(111).36 This BE difference may indicate a different (weak) interaction mechanism of PEN on both gold surfaces. The slight increase of the BE of C 1s of HEX compared to PEN is hardly understandable by properties of the molecule itself. Similar to the trend of BEs of smaller acenes,36 one might expect a decrease of the C 1s BE of HEX in comparison to PEN due to the increase of the -conjugated system accompanied with a change of Fermi energy due to the lowering of the HOMO-LUMO gap. Therefore, we ascribe the slightly different C 1s BE for HEX and PEN to the different energy level alignment to the substrate in both cases, possibly affected by the complex interaction mechanism (see below) at these interfaces.


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a)

C6

0.8 0.6

C2 C1

284

283

282

285 284 Binding energy (eV)

283

282

C5

284.50

Experiment Background C4, C6 C2, C1 C3, C5, C7 Satellite Sum

1.0

C4 C3

C7

0.4 0.2 0.0 287

286

287

286

b)

285

PEN/Si Wafer (h =1487 eV) HEX/Au(110) (h = 1487 eV) HEX/Au(110) (h = 385 eV)

5.3

7.7

Intensity (arb. units)

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


10.3

2.2

1.9 5.6 4.0

16

14

12

10

8

6

4

2

0

-2

Relative binding energy (eV)

Figure 1. (a) Hexacene molecule with different carbon species (top) and C 1s core level spectrum of a hexacene multilayer film (bottom), taken at hν = 1487 eV with detailed peak fit components and residuals. (b) Zoom into the satellite region of C 1s core level spectra of hexacene and pentacene in thin films, taken at different excitation energies.

The different interaction on Au and Ag interfaces may explain different C 1s binding energies reported in the literature.36-37 Generally, in C 1s core level spectra of acenes components arising from carbon atoms with different chemical environments can be distinguished. This was impressively demonstrated for pentacene gas phase spectra.38 However, in thin films the 7 ACS Paragon Plus Environment


interpretation of the peak shape becomes more complicated, most likely due to the influence of intermolecular interaction in the condensed phase.36 The peak shape of the HEX C 1s spectrum in Figure 1 can be described using a model with three different carbon components analogously to experimental gas phase spectra of pentacene.38 In this model, the binding energy of outer C-H carbon atoms (C1 and C2 in Figure 1a, inner C-H (C4 and C6) and carbon atoms without C-H bond (C3, C5 and C7) are distinguished and respective components are weighted according to stoichiometry. We included an additional tiny component at high BE, which might originate from low energy satellite features, possibly. The derived sum curve of the components is in good agreement to the experimental spectrum. The black line at the bottom of Figure 1a represents the residuum of the fit (gray dash lines express standard deviation). The intrinsic satellite structure of the C 1s photoemission spectrum in Figure 1b gives additional insight into the unoccupied electronic states. Such satellite features are caused by a (kinetic) energy loss of the photoelectrons due to simultaneously excited electrons (mostly   * transitions), also called shake up structures. For fullerenes, it was shown that the shake-up spectrum can be compared to the loss function obtained by electron energy-loss spectroscopy.39-40 Due to the different nature of the shake-up process (which allows monopole transitions), also dipole forbidden interband transitions can be identified.39 Whereas for benzene the predicted transitions are in good agreement with the experiment, with increasing molecular size the theoretical description becomes more complicated - due to the number of electronic states, the corresponding shake-up spectra consist of a wealth of peaks and shoulders.36 We compare the C 1s satellite region of HEX at two different excitation energies with PEN in Figure 1b qualitatively. At the photoexcitation energy of hν = 1487 eV, minor differences but very similar features were found for both molecules at 5.3, 7.7 and 10.3 eV, which are in reasonable agreement with reported literature data.36 As a function of excitation energy the peak shape of the HEX satellite spectra changes and in the more surface sensitive spectra recorded using hν = 385 eV two features can be observed at around 5.6 and 4.0 eV, analogously to surface sensitive C 1s spectra of PEN on Ag(111) at hν = 334 eV (5.6 and 4.5 eV).36 The lowest visible feature for HEX is overlapping with the C 1s main peak and a tail might be identifiable at around 1.9 eV. In general lowest lying satellites associated with a HOMOLUMO shake-up process for PEN and HEX are expected to overlap with the main line36 and therefore are not visible in Figure 1b. Generally, the overall shape of the satellite structures of 8 ACS Paragon Plus Environment

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HEX in Figure 1b is typical for higher acenes, indicating that the preparation of HEX multilayer films by thermal evaporation was successful. Compared to lower acenes,36 valence band spectra of HEX and PEN (Figure 2a) show an increasing complexity, which is in line with the increase of molecular orbitals with the number of annulated rings. For both molecules, HEX and PEN, the maximum of the feature with the lowest binding energy (HOMO) is found at 1.0 eV. The HOMO onset for PEN is found at 0.5 eV, comparable to other Au interfaces,36, 41-42 and this value is lower compared to more reactive substrates, such as Ag or Cu.43 For HEX the HOMO onset is observed at about 0.6 eV, indicating a slightly different broadening of the HOMO feature compared to PEN. Small differences in the energetic position, also reflected in the C 1s BEs, can be also caused by a slightly different pinning position of the Fermi level in the gap of the organic semiconductor for PEN and HEX at the Au(110) interface. The energy level diagrams of PEN and HEX on Au(110) are shown in Figure S6 (Supporting Information). We note that the ionization potential and the interface dipole may depend on the orientation of the acene molecules.44 The valence band features at higher BEs are distinctly different for PEN and HEX: whereas a group of features with a lower BE maximum at 2.6 eV is found for PEN, two clearly resolved features at 2.2 and 3.1 eV are found for HEX. The HOMO and the features at 2.2 and 3.1 eV, denoted HOMO-1 and HOMO-2, are also clearly resolved in the high-resolution valence band spectrum of HEX, measured with synchrotron radiation (Figure 2b). The intensity of valence band features might be angle dependent. Therefore, lens modes with high angle acceptance were used. The angle acceptance of the Phoibos 150 (Fig. 2a, homelab) and Scienta SES 200 (Fig. 2b, KARA) analyzers was estimated to be about 15°. We note that the 30° angle integrated spectrum measured at a photon energy of 40.8 eV (Figure S3b) shows an excellent agreement of the spectral shape for lowest lying valence band features of HEX. At the excitation energy of 140 eV the relative cross section of C 2s is increased with respect to C 2p valence electrons.28 Generally, the positions of valence band features are in good agreement with energies of molecular orbitals (MO) obtained from DFT calculations, added as black bars with Gaussian broadening in Figure 2b (the HOMO energy was set to 1.05 eV). In addition, the calculated HOMO-LUMO gap (1.8 eV for HEX and 2.2 eV for PEN) is in good agreement with recently published experimental data probed with scanning tunneling 9 ACS Paragon Plus Environment


spectroscopy (1.85 eV for HEX and 2.2 eV for PEN).17 Recently, very good agreement was obtained by comparing quasiparticle calculations and experimental photoemission data for pentacene.45 Further, a systematically increasing difference was reported between calculated quasiparticle and DFT energies with increasing MO energies.45 As a consequence, in order to align calculated with experimental spectra, usually not only a shift-factor (correction of the well-known DFT bandgap error), but also a stretch factor is applied. Our ground state calculations do not include these advanced features of the quasiparticle approach and therefore they certainly underestimate binding energies of the higher MOs. Therefore we propose for structures at binding energies of more than 4 eV a tentative assignment only.

h = 21.2 eV 9.2

b)

3.1 eV 2.2 eV

Hexacene

h = 140 eV

1.0 eV

8.1 3

7.1

2

1

0

Pentacene Hexacene

3.1 2.2


a)


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

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HOMO-2 HOMO-1 HOMO LUMO

1.0

10

8

6

4

2

14 12 10

0

Binding energy (eV)

8

6

4

2

0

Binding energy (eV)

Figure 2. Valence band spectra of 6 nm thick films taken at normal emission: (a) Hexacene compared to pentacene excited with He I radiation (h = 21.2 eV). (b) Hexacene valence band spectrum excited using synchrotron radiation (h = 140 eV) and calculated MO energies (DFT, B3LYP/6-31G*) with Gaussian broadening (FWHM 0.5 eV).

Unoccupied Electronic Structure and Molecular Orientation in Thin Films. In particular for well-ordered organic molecules in thin films, the angular distribution of the photoemitted electrons can give valuable information about the electronic structure.46-49 As an example, the energy-band dispersion due to the intermolecular  interaction is related to the transfer integral and thus to the hole mobility.49 For molecules uniaxially oriented on the substrate 10 ACS Paragon Plus Environment

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surface, even real space molecular orbitals can be reconstructed from photoemission data.47 For HEX we do not observe an unambiguous dispersive behavior, which suggests that the molecules are not strongly oriented (see Supporting Information Figure S3a). C K XAS spectra of a about 4-5 nm thin HEX film on Au(110) are shown in Figure 3 for three different angles of the incoming p-polarized synchrotron light. Generally, at lower photon energies, transitions into * orbitals are observed, whereas at higher photon energies (> 290 eV) transitions into * orbitals are dominating. At energies < 290 eV two groups of features can be distinguished (283.6/284.3 and 285.6/286.2 eV).

C K edge 286.2 284.3  = 90°

intensity (arb. units)

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


 = 45°

 = 20°

280

290

300

310

320

Photon energy (eV)

Figure 3. C K edge XAS spectra of a hexacene film grown on Au(110) as a function of the angle  between the surface normal and the electric field vector of the p-polarized synchrotron light (see inset). The lower energy features (< 290 eV) represent the * resonances, whereas those features above 290 eV are related to * resonances.

For the related PEN molecule, features at lower photon energies were attributed to excitations into the LUMO, while the resonances in the energetic region of 285–287 eV correspond to excitations into the second lowest unoccupied molecular orbital.38,

50

Analogously, for HEX we distinguish two features below 285.0 eV and further (or additional) two sharp features above 285.0 eV. Broad features at photon energies higher than 290 eV can 11 ACS Paragon Plus Environment


be attributed to transitions into * orbitals. For planar -conjugated molecules, the intensity of C 1s-* excitations can be used to analyze the molecular orientation.51-52 If the electric field vector of the incoming synchrotron radiation is oriented parallel to the π* orbital, the absorption is maximal, whereas the transition is forbidden in the case of a perpendicular orientation. Alternatively, transitions into σ* orbitals can be monitored with opposite angular dependence. The intensity of the C 1s-* excitations in Figure 3 is almost independent on the polarization of the incoming light, indicating the absence of a preferred flat lying or “edge-on” molecular orientation. This might be due to formation of the typical herringbone packing motif of crystalline acene structures, including hexacene.9 Electronic Interface Properties on Au(110). Electronic interface properties of HEX were studied on the comparably inert Au(110) substrate using PES and XAS. Information about interactions of HEX at the interface to gold might be obtained from thickness dependent C 1s core level spectra. However, due to the overlapping contributions from different carbon species, changes of the peak shape are hardly visible. Nevertheless, distinct changes in the C 1s satellite region as a function of the thickness are displayed in Figure 4 with relative binding energies to the main peak. We discuss surface sensitive spectra taken at hν = 385 eV.

h = 385 eV 6 nm 0.7 nm 0.4 nm


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

14

12

7.7

10

8

5.6

6

4.0

1.9

4

2

0

Relative binding energy (eV)

Figure 4. Satellite region of hexacene C 1s core level spectra of as a function of the thickness, taken at hv = 385 eV.

Most important, almost all satellite features at 7.7, 5.6, 4.0 and 1.9 eV (already discussed above, see also Figure 1b) disappear as the thickness of the film decreases to one monolayer 12 ACS Paragon Plus Environment

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(0.4 nm) as can be seen in the spectra displayed in Figure 4. Such changes indicate an electronic interaction between the HEX monolayers and the substrate Au(110).53-54 Thus, these observations give a first hint for an electronic coupling of the aromatic π-system with the Au(110) substrate. Further indication for interactions at interfaces might be extracted from valence band spectra as a function of the film thickness. In contrast to valence band spectra of multilayer films (Figure 2), spectra recorded at lower HEX coverages as shown in Figure 5 are increasingly dominated by substrate related features of Au(110). In the low BE region additional intensity around 0.8 eV is clearly visible after evaporation of HEX monolayers (Figure 5b).

a)

b)

h = 140 eV

hv = 140 eV

9.1 7.0 8.1

3.1 2.1

Film

1.0 EF

0.7 nm substrate



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


0.8

0.7 nm

substrate

14

12

10

8

6

4

2

Binding energy (eV)

0

2

1 0 Binding energy (eV)

-1

Figure 5. (a) Valence band spectra of hexacene on Au(110) as a function of the thickness, taken at hν = 140 eV. (b) Zoom into the region of low binding energy. Dashed vertical bars indicate positions of hexacene derived spectral features.

Besides small energy shifts towards lower binding energies, which might be caused by the enhanced polarization screening at the metal interface, the typical features of HOMO at 0.8 eV and possibly HOMO-1 at about 1.7 eV (see arrow in Figure 5b) can be identified. However, additional intensity close to the Fermi level, e.g. due to the formation of so-called “gapstates”, cannot be completely excluded due to the broad shape of the HOMO, which may point 13 ACS Paragon Plus Environment


to an interaction at the interface. However, the exact analysis depends strongly on the details of the subtraction of the Au background signal. We note, that a strong interaction might be visible in C1s core level spectra, as shown at the example of acene/Cu surfaces.55-57 The interaction between HEX and Au(110) seems to be weaker, since we do hardly see any change in the C1s photoemission spectra (Figure S4). The lowering of the C1s binding energies for coverages in the monolayer range in Figure S4 is common for organic molecules on metal interfaces; the size of 0.3-0.5 eV can be understood by core-hole screening effects.58 Thus, the C1s spectra alone provide no clear evidence for a strong interaction or band bending at this interface. Further information about interactions at interfaces can be gained from XAS. Figure 6 shows the carbon K edge XAS spectra of thin HEX films on Au(110) with thicknesses of 1.5 nm and 0.8 nm for different incidence angles of the p-polarized light. As discussed above, the origin of the lower lying features might be ascribed to transitions of the inequivalent carbon atoms into the LUMO and LUMO+1.

a)

0.8 nm

C K edge

b)

284.5

*

*

 = 90°

 = 45°

Total electron yield (arb. units.)

286.0

Total electron yield (arb. units.)

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

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C K edge

1.5 nm

286.0 284.6

*

*

 = 90°

 = 45°

 = 20°

280

290

300

 = 20°

310

280

Photon energy (eV)

290

300

310

Photon energy (eV)

Figure 6. Angle dependent C K edge XAS spectra for 0.8 nm (a) and 1.5 nm (b) coverages of hexacene on Au(110). The peak shape is significantly changed compared to the thicker film (see Figure 3).


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The shape of the XAS spectra for few monolayer coverages (Figure 6) and the thicker film (Figure 3) differ significantly. It seems that the splitting within the two groups of * features is not present anymore and the energetically low lying structures of * resonances at 284.6 (1.5 nm) and 284.5 eV (0.8 nm) are shifted compared to the thicker film (Figure 3). We note that XAS spectra of coverages lower than 2 ML (see Supporting Information, Figure S4) are very similar to spectra of 0.8 nm, but the latter are less well resolved due to the weak signal to noise ratio. The origin for this difference in peak shape compared to spectra from thicker films is suggested to be a strong electronic coupling of the molecular states of HEX with states of the substrate, similar to PEN on Au(111)50 and Ag(111)59. These results confirm the assumption of an interaction at this interface discussed above. Molecular Orientation in the Monolayer Range. Comparing the ratio of intensities for C 1s* and C 1s-* excitations in Figure 6, a stronger dependence on the polarization of the incoming light can be observed compared to XAS spectra of the thicker film (Figure 3). At normal incidence (90°) * features are maximal, especially for low coverages (0.8 nm), whereas transitions into * exhibit lower intensity. At grazing incidence (20°) the intensities of * and * resonances show an inverse behavior for these low coverages and strong transitions into *-orbitals can be observed. The stronger anisotropy of the spectra for low coverages indicates that HEX molecules adsorbs in a preferred lying orientation on the Au(110) substrate, similar to PEN on Au(111)41-42 and PEN on Au(110).23-24 Pentacene molecules first adsorb on Au(110) in a head-to-head orientation within the gold channels of the (110) substrate. Once the gold channels have been filled up, adsorption occurs on top of the gold rows with the molecules adopting a side by side orientation.23 Assuming a similar adsorption geometry for HEX/Au(110) and PEN/Au(110) the weaker dichroism in thicker films would point to larger tilt angles parallel to molecular stacks, since the electric field vector was varied while keeping the azimuth in [1-10] direction of the Au(110) single crystal. The larger tilt angles found in thicker films can be understood in this manner by intermolecular interactions, which become dominant in thicker films. In thick films this might result in a typical bulk-like herringbone structure analogously to pentacene.60



CONCLUSIONS We successfully synthesized hexacene by a modified Meerwein-Ponndorf-Verley reduction of hexacene-6,15-quinone, evaporated hexacene and grew films on Au(110) for the first time by physical vapor deposition. This procedure allows obtaining hexacene films with variable thickness, while on-surface preparations typically result in low coverages.17 The film growth was monitored using photoemission and x-ray absorption spectroscopy (XAS). Both the shape of the C 1s core level spectrum with its satellite structure and C 1s excitation spectra are similar to pentacene and characteristic for an extended aromatic system. Valence band features of the HOMO, HOMO-1 and HOMO-2 agree well with the density of states obtained from DFT, while well resolved peaks in XAS are comparable to pentacene and can be assigned as transitions into LUMO and LUMO+1. In particular thickness dependent peak shape changes in XAS indicate interactions between hexacene at the interface to Au(110). In addition, polarization dependent XAS spectra reveal changes in the molecular orientation as a function of the film thickness. Whereas the hexacene molecules prefer an almost flat lying orientation on the Au(110) substrate in the monolayer range, larger tilt angles can be found in thicker films. This study shows that the preparation of well-defined thin films of hexacene is possible and that the electronic properties are supportive of semiconducting behavior. This may open the way for hexacene to be a potential candidate for organic semiconducting devices.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications Website. Synthetic details, C 1s peak fit of pentacene, Band map of hexacene film, C1s photoemission spectra of hexacene on Au(110), XAS spectra of 1-2 monolayers of hexacene on Au(110), Energy level diagrams of hexacene and pentacene on Au(110). (PDF)

AUTHOR INFORMATION Corresponding author. *E-mail: [email protected], Phone: (+49) 07071 / 2976931, Fax: (+49) 07071 / 29-5490 16 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS We thank the Helmholtz Zentrum Berlin (HZB), as well as the synchrotron light source KARA and the KNMF (both Karlsruhe, Germany) for the allocation of synchrotron radiation. Financial travel support by HZB is thankfully acknowledged. This research was funded in part by the Deutsche Forschungsgemeinschaft. We thank Hilmar Adler and Erika Giangrisostomi for helpful discussions and technical support.

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