Ordered Growth and Electronic Properties of 1,2:8,9-Dibenzopentacene

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ordered Growth and Electronic Properties of 1,2:8,9Dibenzopentacene (Trans-DBPen) on Ag(111) Felix Otto, Tobias Hümpfner, Maximilian Schaal, Christian Udhardt, Lennart Vorbrink, Bernd Schröter, Roman Forker, and Torsten Fritz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00095 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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The Journal of Physical Chemistry

Ordered Growth and Electronic Properties of 1,2:8,9-Dibenzopentacene (trans-DBPen) on Ag(111) Felix Otto, Tobias Huempfner, Maximilian Schaal, Christian Udhardt, Lennart Vorbrink, Bernd Schroeter, Roman Forker, and Torsten Fritz∗ Institut f¨ ur Festkörperphysik, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany E-mail: [email protected]

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Abstract We study highly ordered two-dimensional layers of 1,2:8,9-dibenzopentacene (transDBPen) adsorbed on a single-crystalline silver surface. While its parent molecule pentacene is known for a relatively high hole mobility, reports on trans-DBPen are rather scarce.

Furthermore, it belongs to the polycyclic aromatic hydrocarbons

(PAHs) for which superconducting properties have been observed upon intercalation with potassium.

Our scanning tunneling microscopy (STM) and low-energy

electron diffraction (LEED) results reveal a highly ordered monolayer structure of trans-DBPen on Ag(111). We further used angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) to measure photoelectron momentum maps (PMMs). The experimental PMMs agree very nicely with simulations based on our structural data while assuming free molecules for the density functional theory (DFT) calculations. A comparison with pentacene yields some insights into the properties of the two related molecules. We conclude that the degree of hybridization between the molecular orbitals and substrate states is comparatively weak. These results are expected to serve as a starting point for future investigations of K-doped monolayers of trans-DBPen.

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Introduction The polycyclic aromatic hydrocarbon (PAH) 1,2;8,9-dibenzopentacene (trans-DBPen) is a derivative of pentacene (Pen), which has been the subject of many investigations. Pen is used in organic electronics (e.g., field effect transistors), because its hole mobility is relatively high compared to other organic molecules. 1,2 Beyond that, the adsorption of Pen on various substrates is a well-investigated model system as a result of its simple shape. 3–5 TransDBPen is also known for exceptional electronic properties, because it belongs to the group of superconducting PAHs if doped with alkali-metals. 6–9 Up to now there are only few studies regarding 1,2:8,9-dibenzopentacene (trans-DBPen, see fig. 1). Among them are theoretical calculations concerning the mechanism of superconductivity and the bulk structure of trans-DBPen. 10–12 Others are electron spectroscopic measurements in a thickness range of 10 monolayers (ML) up to 100 nm to reveal the behavior upon potassium doping and the electronic properties compared to the parent molecule pentacene for which no superconducting phase has been found. 13–15 In our study we focus on the relation between structure and electronic properties of ultrathin films of trans-DBPen on Ag(111). For this purpose we combined low energy electron diffraction (LEED), scanning tunneling microscopy (STM) as well as x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS). We observed a highly ordered growth beyond a film thickness of 1 ML and could measure photoelectron momentum maps (PMMs) which are comparable to simulations based on free-standing molecules. All our results consistently suggest a comparatively weak electronic interaction between the molecules and the Ag(111) substrate.

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Experimental and Computational Methods Experimental Methods Photoelectron spectroscopy (PES) was performed with a surface analysis system (SPECS Surface Nano Analysis GmbH) at room temperature within an ultrahigh vacuum (UHV) chamber with a base pressure < 2 × 10−10 mbar. XPS was performed using photons originating from monochromatized Al Kα excitation (Focus 500, 1486.71 eV). The He Iα (21.218 eV) emission from a microwave-heated gas discharge lamp (UVLS) together with a toroidal mirror monochromator (TMM 304) adjusted in p-polarization was used for the UPS measurements. The angles between the radiation sources and the analyzer were 55◦ for XPS and 50◦ for UPS. The photoelectron yield was recorded with a PHOIBOS 150 hemispherical electron analyzer and a 3D DLD4040-150 delay-line detector. The effective energy resolution was about 50 meV. The secondary electron cut-off (SECO) was measured by applying a bias of ≈ −9 eV, which was determined separately for each film thickness. The electron acceptance angles of the analyzer for XPS and UPS were ±8◦ and ±7◦ , respectively. PMMs were acquired by tilting the sample from −10◦ to 80◦ in steps of 2◦ (polar angle θ) and rotating from 60◦ to −60◦ in steps of 15◦ (azimuthal angle ϕ). The data set was recorded using the 2D imaging capability of the detector (101 x 50 channels) and processed with a home-made software as described in ref. 16. This includes dividing by the cosine of the polar angle to account for the electron emission characteristics. The PMMs were integrated in a range of ±0.1 eV around the denoted energy to improve the signal-to-noise ratio. Qualitative LEED measurements were performed with a 4-grid MCP2-SPECTALEED with two microchannel plates (Omicron NanoTechnology GmbH) at room temperature. The quantitative determination of the highly ordered structure was carried out with a similar device, namely a BDL800IR MCP2 (OCI Vacuum Microengineering). The correction of distortion and evaluation of the data was performed with the commercial softwares LEEDCal 17 and LEEDLab 18 as described elsewhere. 19 STM was performed using a JT-STM/AFM (SPECS

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Surface Nano Analysis GmbH) at 1.1 K with an Ar+ -sputtered tungsten tip. All images shown in this paper were recorded with a positive bias, which means tunneling from occupied states of the tip to unoccupied states of the sample. A home-made ultrahigh vacuum (UHV) shuttle with a base pressure of about 10−10 mbar was used to transfer the sample between the two different setups used (for electronic and structural characterization, respectively).

Computational Details The orbitals of the free-standing molecules and their energy distribution were calculated via density functional theory (DFT) using Gaussian09 with the hybrid-functional B3LYP and the 6-31G(d) basis set. 20 PMMs were simulated as Fourier transforms of the 3D orbital density using the algorithm described in ref. 21. We are thereby taking advantage of the nearly ~ ~ constant angle-dependent factor A · k of the photoemission intensity in our experimental setup.

Materials and Thin Film Deposition The Ag(111) single crystal (Mateck GmbH, purity 99.999 %) was prepared by repeated Ar+ sputtering (700 V, 4 µA, 4 × 10−5 mbar, angle of incidence 60◦ , sample rotated during sputtering) and annealing in UHV (≈ 500 ◦C, 30 min). The quality of this process was checked via LEED, UPS and XPS. Dibenzopentacene (DBPen) was purchased from TCI Deutschland GmbH. There are two isomers of DBPen: 1,2:8,9-dibenzopentacene (trans-DBPen, CAS registry number 227-098, fig. 1 left) and 1,2:10,11-dibenzopentacene (cis-DBPen, 227-07-6, fig. 1 right). The differences between the two isomers will be discussed later. We use the unit monolayer equivalent (MLE) to describe the film thickness which refers to the amount of deposited molecules compared to the first closed wetting layer. Why we speak of MLE rather than ML will become apparent later. 5



cis-DBPen Evac

trans-DBPen

Pentacene

0 -1 -2

Energy [eV]

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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LUMO

LUMO

LUMO

HOMO

HOMO

HOMO

HOMO-1 HOMO-2

HOMO-1

HOMO-1 HOMO-2

-3 -4 -5 -6

HOMO-2 -7

Figure 1: Density functional theory (DFT) calculations of the molecular orbitals and their respective energies of 1,2:10,11-dibenzopentacene (cis-DBPen, left), 1,2:8,9-dibenzopentacene (trans-DBPen, center) and pentacene (Pen, right). The red line indicates the degenerate HOMO-1 of trans-DBPen. The HOMO and LUMO of trans-DBPen and the orbitals of Pen agree with previous calculations. 14,22,23 Trans-DBPen was temperature gradient sublimated twice with a setup described in ref. 24 and degassed in UHV before usage. The molecules were deposited from an effusion cell at ≈ 200 ◦C with a deposition rate of about 0.1 MLE/ min and the substrate held at room temperature (base pressure < 3 × 10−10 mbar). The film thickness was determined using a combination of a quartz crystal microbalance and the occurrence of the highly ordered structure in LEED (= 1 MLE) with an error of ±0.1 MLE. The results were checked using the C 1s to Ag 3d ratio in XPS and the work function measurements in UPS.

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(a)

50 nm

2 nm (b)

(c)

−1

−1

0.5 ˚ A

0.5 ˚ A

Figure 2: (a) STM image of 0.5 MLE trans-DBPen on Ag(111) (V = 0.9 V, I = 50 pA). The wide-range scan shows large domains of disordered molecules. Inset: molecularly resolved trans-DBPen molecules (V = 1.4 V, I = 50 pA). (b) Fast Fourier transform of the survey scan in (a). (c) The LEED image of 0.5 MLE trans-DBPen on Ag(111) acquired with a beam energy of 14 eV shows the same pattern as (b).

Results and Discussion Structure In the first part of this section we present our results on the adsorption behavior of transDBPen on Ag(111) as a function of the film thickness. The sub-monolayer regime is characterized by an emerging disc-like LEED pattern as shown in fig. 2(c). This correlates with a minimum distance between the molecules and indicates an effectively repulsive moleculemolecule interaction. Furthermore, the disc has a hexagonal perimeter which suggests a preferential orientation of trans-DBPen with respect to the Ag(111) surface. An STM image with a wide-range scan of a sample with a coverage of 0.5 MLE is shown in fig. 2(a). The molecules form large disordered domains and seem to be arbitrarily aligned. The fast Fourier transform of the image in fig 2(a) shows the same hexagonal disc-like shape 7



(a)

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(b)

~a2 ~a1

2nd ML

(c) 2nd ML ~a2 ~a1

1st ML ~a2 1st ML

~a1 substrate step

(d) ~es1 ~es2

5 nm

~a2 ~a1

Figure 3: (a) LEED measurement of 1.3 MLE trans-DBPen on Ag(111) at a beam energy of 32.5 eV, cooled with liquid helium during the acquisition and contrast inverted. Red: direction of substrate lattice, green: fitted adsorbate lattice including multiple scattering. See text for further explanations. (b) and (c) STM images of 1.3 MLE trans-DBPen on Ag(111) with the same lateral scale (V = 1.4 V, I = 20 pA). The color code in (b) highlights the 1st ML in purple and green (on a substrate step) and the 2nd ML in ocher. Note that (c) shows a different rotational and mirror domain of the 2nd ML than (b). (d) illustrates a model of the structure with the adsorbate unit cell in green. The vectors ~es1 and ~es2 represent the directions of the substrate lattice vectors. as the aforementioned LEED pattern (fig. 2(b)). Consequently, the adsorption behavior of sub-monolayer deposits of trans-DBPen on Ag(111) results in a disordered structure instead of a condensed phase, which has been described by others as a two-dimensional gas. 25,26 The inset in fig. 2(a) shows a zoom into an area where single molecules can be seen. They are not ordered but clearly identifiable as trans-DBPen molecules. It is essential for the investigation of the superconducting properties to distinguish between the two isomers which could be achieved by direct imaging in STM (see supporting information fig. S1). Slightly beneath one MLE we get a superposition of the disc-like LEED pattern shown in fig. 2(c) and a new highly ordered structure which is the same as for the full monolayer coverage (see supporting information fig. S2). The molecules condense into a highly ordered structure upon the increasing pressure induced by neighboring molecules on the substrate surface. Fig. 3(a) shows the characteristic LEED pattern of 1 MLE. The red lines indicate 8


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the direction of the substrate lattice vectors. The LEED spots visible in fig. 3(a) were numerically fitted including multiple scattering and the following epitaxy matrix of transDBPen on Ag(111) was determined: 

 3.00 ± 0.01 2.50 ± 0.01 ˆ = M   . −4.00 ± 0.01 3.52 ± 0.01

(1)

Here, the errors given are purely statistical errors from the fit (1 σ confidence level). The expected spot positions are marked by green circles in fig. 3(a). The two adsorbate lattice vectors have a length of (8.04 ± 0.07) ˚ A and (18.8 ± 0.2) ˚ A, respectively, and enclose an angle of (101.1 ± 0.1)◦ . The domain rotation angle, defined by the angle between the respective first lattice vectors of the adsorbate and the substrate is (51.0 ± 0.1)◦ . For coverages higher than 1 MLE the LEED pattern does not change, while the LEED images itself show an increased background due to inelastic scattering processes. The highest coverage investigated here is 15 MLE. Due to the measurement uncertainties there are two possibilities to interpret the epitaxy matrix in terms of lattice epitaxy as described in refs. 27 and 28. Only the first column consists of integers, which suggests a point-on-line (POL) epitaxy of the adsorbate structure. However, a higher order commensurate (HOC) structure would also be feasible if the second column is regarded to represent half-integer values. This scenario is only slightly out of the statistical error margins given. Consequently, we have to conclude that the two epitaxy types cannot be distinguished by the LEED measurements in this case, and that therefore POL and HOC epitaxy are equally likely here. Accordingly, there is no substantial justification for an adsorbate supercell which might be commensurate with the substrate. Therefore, no attempt is made to indicate the adsorption sites of the molecules in fig. 3. We further performed STM measurements on a sample with a nominal coverage of 1.3 MLE (see fig. 3(b) – (d)). In a 20 x 20 nm2 scan (fig. 3(b)) three domains of different height can be distinguished. The regions in purple and green, which both belong to the 9



1st ML, are separated by a step edge of the substrate. Fig. 3(d) shows a model of the highly ordered structure of trans-DBPen on Ag(111). The long molecular axis, which is equal to the pentacene (Pen) backbone, is aligned to one lattice vector of the adsorbate unit cell shown in fig. 3(d) in green. We did not detect uncovered substrate area. The ocher region in fig. 3(b) could be identified as the second ML of trans-DBPen on Ag(111). The molecules exhibit a different shape compared to the first ML (compare fig. 3(b) and (c)). This can be interpreted as a submolecular contrast which arises due to a weakened electronic interaction between the 2nd and the 1st ML compared to the one between the first monolayer and the substrate. Noteworthingly, the growth of trans-DBPen thin films on Ag(111) differs from the adsorption of pentacene on Ag(111). In the case of pentacene the first monolayer exhibits no highly ordered structure and is described as “liquid-like” due to the absence of clear LEED spots. 4,29 The situation is comparable with the sub-ML regime in the system transDBPen/Ag(111) in our work. In contrast, the 2nd ML of Pen on Ag(111) exhibits a highly ordered structure.

Influence of Temperature and Pressure The following section focuses on the annealing behavior of trans-DBPen films on Ag(111). In all cases the sample was held at the given temperature for about 30 min. For those investigations we used samples with a nominal thickness of 2 MLE that exhibit a highly ordered structure as already discussed above. The LEED pattern does not change upon heating up to 160 ◦C. At about 170 ◦C we reach the intermediate range where the monolayer structure and the 2D gas-like disc are visible via LEED (see supporting information fig. S2). After exceeding a temperature of 180 ◦C the characteristic hexagonally shaped disc is observed solely which suggests further desorption of molecules (supporting information fig. S2). The desorption of the 1st ML takes place in a rather broad temperature range. We also investigated the influence of exposure of 2 MLE films to air and nitrogen at 10


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atmospheric pressure for 40 min. Afterwards the LEED patterns of those samples no longer exhibit any discernible spots. Surprisingly, slight annealing at about 150 ◦C was sufficient to restore the original pattern shown in fig. 3(a). This behavior is quite remarkable.

Electronic Properties At first we discuss the changes in the UP spectra upon variation of the film thickness shown in fig. 4(a). The electron emission angle of 70◦ was chosen because it results in higher surface sensitivity owing to the limited inelastic mean free path of the photoelectrons escaping the surface. Furthermore, this emission angle was found to result in a satisfying intensity of the different molecular orbitals with respect to the substrate signal. The Fermi edge is not vanishing even at the highest coverage we investigated (15 MLE), which is quite remarkable considering the electron escape depth in the order of 1 nm. A diminishing sticking coefficient for higher thicknesses as explanation is implausible, as the C 1s signal is still increasing compared to the Ag 3d intensity with increasing thickness (see fig. 4(b)). We can therefore conclude, in combination with the STM result of the closed 1st ML, that the thin film growth mode is Stranski-Krastanov. There is one wetting layer of flat-lying molecules and islands on top. That is why we use the unit monolayer equivalent to emphasize that we do not have layer-by-layer growth. This is highly comparable to Pen on Ag(111). 4,30 The Shockley surface state (SS) of Ag(111), which is only visible in normal emission, vanishes for deposited amounts between 0.3 MLE and 0.5 MLE of trans-DBPen (supporting information fig. S3). On the one hand there is the possibility that the SS is suppressed due to the adsorption of the molecules. On the other hand it may be unoccupied and therefore shifted above the Fermi level. The latter scenario occurs for 1 MLE of xenon on Ag(111). 31 In the sub-ML range the Shockley surface state is retained at exactly the same position as for the bare substrate and is visible up to nearly 1 MLE. 32 Xenon forms well ordered domains even without forming a closed wetting layer. 32 Uncovered areas of the substrate remain, which therefore show the Shockley SS of Ag(111). In contrast, trans-DBPen shows a 2D 11



(a ) H O M O -2 H O M O -1

In te n s ity [a .u .]

H O M O

4 .0

1 5 5 .0 3 .5 2 .0 1 .0 0 .8 0 .5 0 .3 A g 3 .5

3 .0

2 .5

2 .0

1 .5

1 .0

0 .5

0 .0

M L E M L E M L E M L E M L E M L E M L E M L E (1 1 1 )

-0 .5

-1 .0

B in d in g e n e r g y [e V ]

1 s / A g 3 d

(b )

1 5 5 .0 3 .5 2 .0 1 .0 0 .8 0 .5 0 .3

In te n s ity r a tio C

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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2 8 8

2 8 7

2 8 6

2 8 5

2 8 4

M L M L M L M L M L M L M L M L

E E E E E E E E

2 8 3

B in d in g e n e r g y [e V ]

Figure 4: (a) UP spectra (polar angle 70◦ ) in dependence of the trans-DBPen film thickness. The labels HOMO-2, HOMO-1 and HOMO indicate the position of the respective orbitals. (b) C 1s spectra of trans-DBPen on Ag(111) normalized to the Ag 3d intensity acquired at normal emission. gas-like behavior in the sub-ML regime that covers the substrate surface homogeneously as discussed above. A more or less continuous shift of the SS toward the unoccupied region is expected. Something similar was observed for Na, which adsorbs on Au(111) in a 2D gas-like layer. 33 This results in a uniform charge transfer to the substrate and a continuous shift of the SS in energy. 33 The UP spectrum of 1 MLE exhibits two clear features at a binding energy (BE) of 3.0 eV and 2.0 eV which can be assigned to emission from the HOMO-1 and HOMO, respectively. Both states shift to higher BE upon increasing film thickness. This effect is caused by the

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emerging interface dipole and can be attributed to the so called push-back effect. 34 Note that the HOMO-1 is two-fold degenerate as also stated by Mahns et al., and in fact its intensity is approximately twice as large as that of the HOMO. 14 Further, the HOMO-2 is discernible below 2 MLE, but gets significantly broadened for higher coverage. In the range between 2.0 MLE and 5.0 MLE no significant peak shifts are observed. Next we discuss the spectrum at 15 MLE. On the one hand all peaks get broadened which is why the HOMO-2 seems to gradually disappear. We assume this to be caused by a growing inhomogeneity, in agreement with the increasing scattering background observed in our LEED experiments. On the other hand the peaks for 15 MLE are shifted towards lower BE. This is counterintuitive at first glance. Lu et al. reported a similar, even more pronounced behavior for the adsorption of Pen on Ag(111) which they attributed to intermolecular interactions and the resulting band formation. 4 There, resolving the dispersion by means of angle-resolved UPS (ARUPS) was hindered by the simultaneous occurrence of equivalent domains (owing to rotational and mirror symmetries of the Ag(111) substrate), which is why it was only possible to measure a broadened intensity distribution in k-space. 4 These arguments most likely also apply to our observations. The C 1s spectra of trans-DBPen on Ag(111) (fig. 4(b)) show a shift to higher BE from 0.3 MLE to 3.5 MLE which can be explained by the push-back effect similar to the shift of the HOMO and HOMO-1 discussed above. Increasing film thickness results in a more pronounced asymmetry of the C 1s state. This can be attributed to the islands growing on top of the wetting layer which contribute to an inhomogeneous film structure. These may result in charging effects which can cause peak shifts. 35 Another consequence of the Stranski-Krastanov growth mode can be seen in the secondary electron cut-offs (SECOs) depicted in fig. 5(a). Up to a nominal film thickness of 2.0 MLE there is a single sharp edge. The spectra for higher nominal film thicknesses exhibit a kink which can be interpreted as a 2nd SECO stemming from different parts of the sample. Thicker films are composed of regions with a coverage of one closed layer of trans-DBPen and areas

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In te n s ity [a .u .]

(a )

1 5 5 .0 3 .5 2 .0 1 .0 0 .8 0 .5 0 .3

1 2 .9

M L M L M L M L M L M L M L M L

E E E E E E E E

1 3 .0

1 3 .1

1 3 .2

1 3 .3

1 3 .4

1 3 .5

1 3 .6

1 3 .7

1 3 .8

K in e tic e n e r g y [e V ]

(b )

4 .6 m a in e d g e 2 nd e d g e

4 .5

W o r k fu n c tio n [e V ]

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4 .4 4 .3

p u s h -b a c k e ffe c t

4 .2 4 .1 4 .0 3 .9

m o le c u la r r e o r ie n t a t io n

3 .8 3 .7 3 .6 0

2

4

6

8

1 0

1 2

1 4

1 6

T h ic k n e s s [M L E ]

Figure 5: (a) Secondary electron cut-offs (SECOs) of trans-DBPen on Ag(111). A bias of ≈ −9 V was applied. The dashed lines are generated by shifting and scaling the 0.5 MLE curve to the cut-off at higher kinetic energy (2nd edge), thereby illustrating the difference with respect to the readily visible main edge (i.e., at lower kinetic energy). (b) The work function was determined through the intersections of the extrapolations of the linear intensity drop with the flat baseline. Values for the 2nd edge were determined in the same way, but for extrapolations of the dashed curves in (a). with multilayer islands. These are likely to exhibit different work functions which readily explains why we observe a superposition of both SECOs. The dashed lines in fig. 5(a) are adapted from the spectrum of the sample with a thickness of 0.5 MLE to illustrate a possible shape of the underlying SECO. The work function (fig. 5(b)) was determined by fitting straight lines to the SECOs at the point of inflection and extrapolating them to the flat background. From 0 to 1 MLE we observe a steep decrease of the work function by ≈ 0.7 eV. This 14


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again can be attributed to the push-back effect of the molecules adsorbing on the substrate and not on top of each other. 36 As a consequence of the two SECOs emerging for higher film thicknesses, also two branches of the work function have to be discussed. The work function of one branch decreases further until it reaches a value of 3.7 eV. The second branch, which corresponds to the SECO of the closed film, results in a nearly constant work function from 2 MLE onwards. One possible reason for the difference between the two work functions is a molecular reorientation from flat-lying to a slightly tilted configuration as proposed for pentacene on Ag(111). 4,36 Other possibilities are effects due to charging or band bending. 35 The ionization energy decreases from 5.6 eV to 5.4 eV which is compatible with our interpretation of tilted trans-DBPen molecules starting to occur somewhere beyond an island height of two layers. Similar cases were already discussed for Pen on Ag(111) and other molecules. 4,37–40

Photoelectron Momentum Maps (PMMs) The PMMs of 1 MLE trans-DBPen on Ag(111) are shown in fig. 6 (a), (b), and (e). The intensity maxima of the HOMO-1 appear rotated by 30◦ with respect to those of the HOMO. For the simulations of the PMMs we consider three rotational and the corresponding mirror domains which are induced due to the substrate symmetry including the structure obtained from the LEED analysis in fig. 3(a). As mentioned before, the pentacene backbone of the trans-DBPen molecules is aligned parallel to the adsorbate lattice vectors. With this analysis we simulate the PMMs (under the assumption of free non-interacting molecules) depicted in fig. 6 (c), (d), and (f). Overall, there is a good agreement between simulations and experiments. However, on closer inspection, it is apparent that all simulations have a sixfold symmetry while the measurements, especially the HOMO and the Fermi surface, exhibit a three-fold symmetry. This is a result of the band structure of the Ag(111) substrate, which is superimposed on the intensity distributions of the organic molecule. The basis of the understanding of the PMMs is the comprehension of the orbital structure. 15



Figure 6: Comparison of measured PMMs of 1 MLE trans-DBPen on Ag(111) ((a), (b), and (e)) with simulations assuming free molecules including the symmetry of the thin film ((c), (d), and (f)). (a), (c) / (b), (d): comparison of the measured PMMs of the HOMO-1 / HOMO with the simulated ones. (e) and (f): PMM at the Fermi energy showing no signs of the trans-DBPen LUMO. As mentioned before, we have to distinguish between two isomers of dibenzopentacene. In a first batch we accidentally received cis-DBPen, while the second batch exclusively contained trans-DBPen (cf. supporting information fig. S1). A particular complication may arise owing to their chemical similarity: DFT calculations reveal that among both DBPen isomers and Pen the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are each almost indistinguishable in energy and more or less located at the pentacene core, which has been shown before for trans-DBPen by Mahns et al. 14 In contrast to the latter, cis-DBPen has no degenerate HOMO-1 and a larger energetic difference between LUMO+1 and LUMO+2 (see fig. 1). As a consequence of the similar properties, cis- and trans-DBPen cannot be discriminated based on their frontier orbitals. Since many other

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basic chemical and physical properties, such as solubility, optical absorption, sublimation temperature, and molecular mass are either virtually or even exactly identical, it is quite difficult to determine which isomer is supplied. However, we were able to clearly identify cis-DBPen and trans-DBPen obtained in different batches by means of molecularly resolved STM (see supporting information fig. S1). The degenerate HOMO-1 of trans-DBPen is composed of two orbitals whose shapes resemble the non-degenerate HOMO-1 and HOMO-2, respectively, of pentacene (see fig. 1). The superposition of them yields a simulated PMM whose main maxima are rotated by 30◦ compared to the PMM of the HOMO. The additional fine structure, faintly visible in the measured PMM of the HOMO-1 (fig. 6 (a)), is not reproduced by the simulations in fig. 6 (c). In retrospect, an influence from the HOMO-2 can most likely be ruled out: On the one hand, the energetic difference of ≈ 0.7 eV is too large to have a strong impact, on the other hand, the main intensity of the HOMO-2 is located at smaller values of kk (see supporting information fig. S4). The remaining probable explanation is that this fine structure may originate from the substrate, but the experimental resolution does not justify an in-depth analysis here. Our measurements show further that the measured PMMs do not change significantly at least in the range from 1 MLE to 15 MLE. There are no apparent signs that would corroborate the assumption of tilted molecules for film thicknesses > 1 MLE. This seems to contradict the above statements. The ionization energy decreases upon an increase of the film thickness, which can be caused by tilted molecules. However, the PMMs give an account of the highly ordered domains of trans-DBPen. We can therefore only conclude from these measurements that no new structure of tilted molecules is formed. We estimate from our calculations that a tilt angle higher than 5◦ would lead to noticeable changes in the PMMs. The influence of an inclination of the molecules was also studied in refs. 41 and 42. The PMM of trans-DBPen on Ag(111) measured at the Fermi energy is shown in fig. 6(e). We observe only the attenuated Fermi surface of the substrate without any additional

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features originating from the molecular thin film. The simulated PMM of the LUMO is depicted in fig. 6(f). The characteristic pattern of the LUMO is easily distinguishable from the PMM of the HOMO due to the rotation of the intensity maxima of 30◦ . Altogether we conclude that there is no significant charge transfer into the LUMO. Nevertheless, the LUMO has to be rather close above the Fermi level. Our DFT calculations determined a HOMO-LUMO gap of 2.2 eV, which is about 0.1 eV higher than the optical gap we measured for 5 MLE thick films on Ag(111) by means of differential reflectance spectroscopy (DRS 43,44 ) (supp. info. fig. S5). As DFT is well known to underestimate the transport gap, we expect the LUMO to lie at least 0.2 eV above the Fermi energy, but most probably at even higher energies. This shift of the Fermi energy close to the LUMO leads us to the conclusion that the interaction between the trans-DBPen molecules and the Ag(111) surface is characterized by physisorption on the verge of weak chemisorption.

Conclusions To summarize, we studied the growth of trans-DBPen on Ag(111) from the sub-ML range up to a thickness of about 15 MLE. A 2D gas-like adsorption was found to occur below 1 MLE. This is characterized by disc patterns in LEED and a vanishing Shockley surface state. After the completion of the 1st ML we identified only one highly ordered structure. The continuing film growth follows a Stranski-Krastanov mode. As a consequence, we observed a splitting of the SECO caused by the wetting layer and the molecules adsorbed in higher layers. The photoelectron momentum maps can be modeled with free trans-DBPen molecules. Furthermore, there is no sign for a charge transfer to the LUMO. This leads us to the conclusion that the electronic interaction between molecules and substrate is rather weak.

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Supporting Information Available STM images of trans- and cis-DBPen. LEED measurements at different nominal coverages and after two steps of annealing. UP spectra acquired in normal emission. Measured and simulated PMM of the HOMO-2 of trans-DBPen. DRS measurement of 5 MLE trans-DBPen on Ag(111).

Acknowledgement This work was financed by the Deutsche Forschungsgemeinschaft (DFG) grants no. FO 770/2-1 and FR 875/16-1. C.U. gratefully acknowledges funding by the Landesgraduiertenstipendium of the State of Thuringia. T.H. gratefully acknowledges funding by the Studienstiftung des deutschen Volkes.

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Ordered Growth and Electronic Properties of 1,2:8,9-Dibenzopentacene

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