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Interplay Between Morphology and Electronic Structure in #-sexithiophene Films on Au(111) Wibke Bronsch, Thorsten Wagner, Sebastian Baum, Malte Wansleben, Kristof Zielke, Ebrahim Ghanbari, Michael Györök, Andrea Navarro-Quezada, Peter Zeppenfeld, Martin Weinelt, and Cornelius Gahl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07280 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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The Journal of Physical Chemistry

Interplay Between Morphology and Electronic Structure in α-Sexithiophene Films on Au(111) Wibke Bronsch,∗,† Thorsten Wagner,∗,‡ Sebastian Baum,† Malte Wansleben,† Kristof Zielke,† Ebrahim Ghanbari,‡ Michael Györök,‡ Andrea Navarro-Quezada,‡ Peter Zeppenfeld,‡ Martin Weinelt,† and Cornelius Gahl∗,† Freie Universität Berlin, Department of Physics, Arnimallee 14, 14195 Berlin, Germany, and Johannes Kepler University Linz, Institute of Experimental Physics, Altenberger Str. 69, 4040 Linz, Austria E-mail: [email protected]; [email protected]; [email protected]

∗

To whom correspondence should be addressed Freie Universität Berlin, Department of Physics, Arnimallee 14, 14195 Berlin, Germany ‡ Johannes Kepler University Linz, Institute of Experimental Physics, Altenberger Str. 69, 4040 Linz, Austria †

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Abstract The properties of organic films are strongly influenced by their growth and structure. In this work, we link the results from three different techniques to elucidate the interplay between the morphology and the electronic structure of α-sexithiophene films grown on Au(111) surfaces. Photoelectron emission microscopy and thermal desorption spectroscopy reveal a two layers thick wetting layer growth at room temperature. On top of the wetting layer the molecules start to form µm-sized crystallites, which are statistically distributed over the surface. This transition in the growth mode is reflected in the electronic structure of the system studied by two-photon photoemission spectroscopy. While in the wetting layer exciton formation is suppressed by the interaction with the metal substrate, Frenkel excitons with picosecond lifetimes are observed in the three-dimensional crystallites.

Introduction Organic semiconductors are important building blocks in modern device fabrication. Nowadays, a large part of displays is already based on organic light emitting diodes (OLEDs). The OLED technology appears promising also for the next-generation of displays, since their mechanical flexibility and low production costs allow for new designs including bendable devices and large active areas. 1,2 In OLEDs as well as in organic solar cells, which represent a second important field of organic-semiconductor-based applications, molecular films are usually embedded in multicomponent systems. 3–6 To ensure charge transport in such devices, it is necessary to create long-lived and mobile excitons in the organic films and to adjust the energy-level alignment at the interfaces between active molecular regions and to the electrodes. Thereby, it is essential to understand the interplay between structural and electronic properties, since generation and decay of excited electronic states in a molecule strongly depend on its local environment. In this context, α-sexithiophene (α-6T) adsorbed on single crystalline noble metal sub2


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strates are model systems to study organic-semiconductor/electrode interfaces. 7–9 Organic thin films do not necessarily grow in a layer-by-layer fashion, but in a Stranski-Krastanov mode where crystallites of different sizes form on top of a wetting layer. 9–12 For α-6T on Au(111), scanning tunneling microscopy revealed that the molecules in the first monolayer adsorb flat-lying on the surface in their all-trans conformation. 7 Large homochiral domains of either left- or right-handed molecules were observed, forming a two-dimensional (2D) lattice with one molecule per unit cell that is incommensurate with the lattice of the underlaying substrate. Varene et al. 8 performed a first two-photon photoemission (2PPE) study on the electronic structure of α-6T films on Au(111). They identified electronic states and observed the ultrafast population of a S1 Frenkel exciton. In this article, we focus on the connection between film morphology and electronic structure depending on the coverage of α-6T on Au(111). We study the coverage range from 1 to 10 monolayers (ML) combining photoelectron emission microscopy (PEEM), thermal desorption spectroscopy (TDS), and time-resolved two-photon photoemission (tr-2PPE). Detailed knowledge of the film morphology turns out to be crucial for the interpretation of electronic excitations in the α-6T films. Our study reveals the formation of a two-layer thick wetting layer followed by the on-top growth of µm-sized, statistically distributed crystallites. In those crystallites, Frenkel excitons with picosecond lifetimes are observed. Since wetting layer (WL) and three-dimensional (3D) crystallites coexist, it is essential to correlate electronic structure with thin film morphology to understand exciton formation in such metal-organic hybrid systems.

Experiment Sample Preparation The experiments reported in this work were carried out in two independent ultrahigh-vacuum chambers. The Berlin setup is specialized for 2PPE measurements, the Linz apparatus for 3



Figure 1: Schematic illustration of an α-6T molecule. PEEM. All measurements were performed on gold single crystals (MaTeck GmbH, 5N,< 0.1◦ ). In both experiments, we followed the same recipe for sample preparation. The Au(111) surfaces were prepared in vacuo by cycles of 10 min Ar-ion bombardment at 800 V and subsequent annealing to 873 K. Heating and cooling rates were set to 1 K/s. The annealing temperature was held for 10 min per cleaning cycle. The molecular films were prepared by evaporation of α-6T (Syncom, purity > 99 %) from the solid phase. The molecule is shown schematically in Fig. 1. During the evaporation the temperature of the crucible in the Knudsen cell was set to 533 K. In all experiments, the gold sample was kept at room temperature during film preparation. In the 2PPE setup, α-6T films were prepared by dosing at a constant flux for a specific time. The molecular flux directed under near normal incidence to the sample was determined with a quartz micro balance at the position of the sample directly before and after film preparation. In addition, film growth and coverage were characterized by TDS. In the PEEM experiments, α-6T was also dosed at a constant flux. During film growth, PEEM images were recorded every 2.1 s. Assuming the sticking coefficient to be independent of coverage, each PEEM image can be related to a specific coverage according to its time of acquisition after opening the shutter of the evaporator. The evaporator was positioned under an angle of 65◦ with respect to the surface normal. PEEM and 2PPE spectroscopy were performed with the sample at room temperature.

Thermal Desorption Spectroscopy For TDS the sample was heated with a constant heating rate of 1 K/s set by a PID controller. To calibrate the α-6T coverage, a series of TD spectra was measured with a quadrupole mass 4


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spectrometer (Pfeiffer HiQuad QMG700, 1 – 1024 amu/e, ion counting electronics) with the ionizer at a distance of ∼ 8 cm from the sample surface. The strongest desorption signal was found at 69 amu/e, a commonly observed fragment of thiophenes. 32 The shape of the TD spectrum of this fragment is identical to that of the parent ion at 494 amu/e (see supporting online material). Therefore, standard quadrupol mass spectrometers, which are not capable of resolving the parent ion, can also be used for TDS of α-6T.

Photoelectron Emission Microscopy The photoelectron emission microscope is attached to an ultrahigh-vacuum system with a base pressure below 5 · 10−10 mbar. The integrated sample stage allows for a nominal lateral resolution below 50 nm. Due to the chosen settings of the electrostatic lens system, the actual lateral resolution is limited to about 150 nm by the pixel resolution of the sCMOS camera used for the image acquisition. Usually, a frame rate of 1 frame per second was chosen, which allows real-time observations during the deposition of the organic thin film. For the photoelectron excitation a super quiet Xe lamp (Hamamatsu) is used. It provides a nearly continuous spectrum with a maximum photon energy of hν = 7.75 eV limited by the quartz glass components used in the optical path. The photon energy is sufficient to excite photoelectrons from the Au(111) substrate as well as from the organic thin film. The recorded PEEM images are maps of the local electron yield.

Two-Photon Photoemission The electronic structure of the α-6T films was investigated by 2PPE. In this pump-probe spectroscopy, the sample is exposed to femtosecond laser pulses with photon energies below the sample work-function to excite electrons from occupied to unoccupied states in a first step and probe the excited electron distribution by photoemission in a second step. Femtosecond laser pulses of (3.11 ± 0.02) eV are produced by second harmonic generation (SHG) from the output of a Ti:sapphire laser system (Coherent, RegA 9050) at a repetition rate of 5



300 kHz. UV pulses with a photon energy of (4.22±0.03) eV are generated by SHG of the visible output of an optical parametric amplifier (OPA) pumped by the same Ti:sapphire laser. The experiments were performed with photon fluxes of j(3.11eV) ∼ 5 · 1019 cm−2 s−1 and j(4.22eV) ∼ 1.5 · 1019 cm−2 s−1 . For time-resolved measurements, the length of the UV beam-path was varied by a motorized delay stage. Electrons photoemitted from the sample were detected with a hemispherical electron analyzer in the low angular dispersion mode (SPECS GmbH, Phoibos 100 equipped with a channel plate detector and CCD camera). The analyzer was operated at a pass energy of Epass = 25 eV allowing for simultaneous detection of electrons in the energy range of E − EF from 4.2 to 7.5 eV. Energy distribution curves shown in this work are integrated perpendicular to the energy-dispersion plane with an angular acceptance of ±2.5◦ around normal emission. During the measurements, a bias voltage of −200 meV was applied to the sample. In time-resolved 2PPE data, a signal background originating from delay-independent monochromatic 2PPE of the individual pulses has been subtracted.

Results Photoelectron Emission Microscopy During Thin Film Growth Figure 2 shows a sequence of PEEM images acquired during the deposition of α-6T on a Au(111) single crystal. Up to a coverage of 4 ML, no sharp features are visible in the PEEM images. However, inhomogeneities appear for 1.7 ML. When the nominal coverage exceeds 4 ML, small bright structures become visible in the PEEM images. Even at a nominal coverage of 9 ML, the diameter of the bright structures remains around 1 µm. We attribute these structures to 3D crystallites formed by α-6T. The sequence of PEEM images acquired during the deposition experiment was further analyzed by extracting the histogram of each image, as exemplarily shown for coverages of 2.3, 4.0 and 9 ML in the inset of Fig. 3. The blue curve in Fig. 3 represents the evolution 6


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0.6 ML

1.2 ML

1.7 ML

2.3 ML

10 µm

4.0 ML

~9 ML

3D

WL

Figure 2: Sequence of PEEM images obtained during the deposition of α-6T on Au(111). The field of view is about 30×30 µm2 . For the photoelectron excitation a Xe lamp (hν ≤ 7.75 eV) was used. The contrast of all images was set to the same value so that differences of the image intensity correspond to changes of the local electron yield. Starting from about 4 ML, small crystallites are visible within the PEEM images. The crystallites (3D) are visible as bright spots, whereas the wetting layer (WL) appears as gray background. of the mean electron yield, i.e., the intensity averaged across the entire field of view of the PEEM image (MEY, left ordinate), the red curve displays its normalized standard deviation (NSD, right ordinate). Since the electron detection system including the micro-channel plates and the camera obeys Poisson statistics, we normalize the standard deviation σ to the √ square root of the mean signal (σ/ MEY). This emphasizes non-uniformities of the thin film morphology and its evolution during growth. The MEY and the NSD overall increase with coverage. Their evolution however shows significant variations in the coverage range up to 4 ML. While the MEY initially increases it exhibits a dip upon completion of the first monolayer. The NSD reaches a plateau at coverage of about 0.5 ML. Before the first layer is completed the NSD slightly decreases. Note, that the minimum of the NSD is reached before the minimum in the MEY. This indicates that some molecules adsorb already in the second layer before the first layer is completely closed. Nevertheless, we define the local minimum in electron yield as a coverage of 1 ML. The nominal coverages given in Figs. 2 and 3 are expressed in multiples of the deposition time to reach this minimum of the MEY. During the deposition of the second layer, the electron yield increases linearly. The NSD exhibits a maximum at about 1.7 ML. Indeed, the corresponding PEEM image shown in Fig. 2 reveals a large heterogeneity in the local electron yield, but no structures with sharp boundaries. We therefore attribute this local intensity variations to fluctuations in the local 7



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Figure 3: Transients of the mean electron yield (blue, left ordinate) and the normalized standard deviation (red, right ordinate) as a function of the deposition time (upper abscissa) for the sequence of PEEM images. The two dashed lines mark the time when the shutter of the evaporator was opened and closed. The dotted line at 4 ML coverage marks the onset of visible crystallite formation. Histograms of the local electron yield for the coverage of 2.3, 4.0, and 9 ML are shown in the inset. At 4 and 9 ML the histogram is asymmetric. A bimodal distribution is fitted with two Gaussian functions for WL and 3D contributions. coverage of the second layer due to the formation of 2D islands. The islands must have a size below the resolution limit of the PEEM measurement estimated to be about 150 nm. In this case, the intensity measured by the camera is given by the weighted average of the emission originating from the first and second layer. Consequently, the histograms taken during the early stage of crystallite growth can still be described by a single, but broader Gaussian function compared to the histogram of the complete layer. The Gaussian distribution narrows again upon completion of the second layer. Lateral heterogeneities fade and the PEEM image has an almost homogeneous intensity distribution throughout the field of view. In particular, we attribute the minimum of the NSD at the nominal coverage of 2.3 ML to the completion of the WL consisting of two layers of α-6T. The slightly increased nominal coverage may indicate that the growth of the first two layers is not totally in a layer-by-layer manner in particular due to kinetic effects. Also a coverage 8


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dependent sticking coefficient or a varying surface density in the layers depending on the actual coverage would have a similar effect. After deposition of ∼ 4 ML of α-6T, 3D crystallites become clearly visible in the PEEM images. At this coverage, the histograms of the local electron yield have changed to a bimodal distribution, seen as an asymmetric peak shape in the inset of Fig. 3. The two contributions, the one from the wetting layer and the one from the crystallites can both be described by Gaussian peaks. The area under these peaks therefore is taken as a measure of how much of the field of view is covered by the WL and by 3D crystallites (red and green Gaussians in Fig. 3, respectively). For the 9 ML film, the best fit to the data results in 60 % of the surface being covered by bright-appearing 3D crystallites and 40 % by the bare wetting layer. The onset of 3D crystallization takes place between 2.3 and 4 ML coverage. The lateral resolution of the PEEM images does not allow to determine the critical coverage more precisely, where Stranski-Krastanov growth sets in.

Thermal Desorption Spectroscopy of Wetting Layer and 3D Islands Complementary to in situ monitoring the growth of α-6T films by PEEM, we characterized the organic films by thermal desorption spectroscopy. TDS confirms that three different types of molecular environments are present in thin α-6T films on the Au(111) surface. Figure 4(a) shows a series of TD spectra for different initial surface coverages. In Fig. 4(b), the temporal integral of desorption rates for this TDS series is related to the respective α-6T dosage determined with a quartz micro balance during film preparation. We find a linear dependence of the total coverage on dosage indicating a constant sticking coefficient in this coverage range. However, extrapolation of the data to zero desorption signal reveals an offset in the applied total dose corresponding to a finite amount of α-6T not desorbing intactly. We assign this offset to molecules in the first layer, which strongly bind to the Au(111) surface and do not desorb upon heating to 700 K. According to the signal of the quartz micro balance, the strongly bound α-6T species 9



reaches a density of ρ=(0.56±0.05) molecules/nm2 . This value is slightly lower than the molecular surface density of ρ=0.67 molecules/nm2 of the closed-packed monolayer of α6T/Au(111) determined by scanning tunneling spectroscopy in well-ordered regions. 7 We attribute the difference to TDS averaging over locally ordered and disordered regions exhibiting a lower packing density. We define the coverage of α-6T molecules not desorbing intactly as 1 ML. This definition is expected to coincide within ∼ 10 % with the coverage scale determined by PEEM and thereby enables us to directly correlate the morphology and the electronic structure although measured in different UHV systems.

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Figure 4: Thermal desorption of α-6T from Au(111): (a) TD spectra of α-6T films for different initial coverages prepared at room temperature. The QMS signal is recorded at m/q=69 amu/e and plotted semi-logarithmically to display spectra of wetting layer and multilayer on a common scale. (b) Integrated TD signal as a function of molecular dosage determined by the quartz micro balance during film preparation. The x-offset of the linear dependence is used to define a nominal coverage of 1 ML on the right ordinate. The TD spectra displayed in Fig. 4(a) exhibit two main features: 1.) a peak of molecules desorbing in the temperature range of 450 to 500 K with its maximum shifting from 470 K towards higher temperatures with increasing initial coverage and 2.) for coverages above 2 ML

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an additional peak with a common leading edge for all coverages and rising exponentially with temperature. With increasing initial coverage the steep drop of the latter desorption signal shifts towards higher temperatures. For more than 3 ML the two features overlap in temperature and therefore cannot be disentangled. The second feature in the TD spectra is characteristic for a zero-order desorption. 13 This is indicative of the overall surface density of molecules exposed to the vacuum being largely independent of coverage as it is the case for closed multilayers but also can occur for desorption from clusters or crystallites. 13–15 The appearance of the zero-order peak for initial coverages > 2 ML confirms that starting from the third layer all molecules have a very similar binding energy. This finding is consistent with the transition between wetting layer and crystallite growth at coverages ≥ 2 ML. For coverages of 4 ML the multilayer desorption peak overlaps significantly with the bilayer desorption. With increasing coverage the TD peak of the bilayer is shifted towards higher temperatures while its peak integral stays almost constant. We attribute this effect to the multilayer crystallites hindering desorption from the underlying bilayer and the refilling of vacancies in the bilayer by a-6T molecules from the crystallites due to their high mobility at elevated temperatures. We conclude that the results from TDS are compatible with the growth and morphology of the α-6T films on Au(111) deduced from our PEEM study.

2PPE Spectroscopy Resolving the Electronic Structure To investigate the electronic structure of the monolayer, the wetting layer and the crystallites, we performed time-resolved 2PPE spectroscopy for four different α-6T coverages of 1, 2, 3, and 10 ML. The photon energy of the pump pulse was set to hν1 = 3.11 eV. According to Varene et al. this photon energy is sufficient to excite Frenkel excitons (FE). 8,16 As a probe photon-energy we used hν2 = 4.22 eV, which is well below the work function of the Au(111) surface for all α-6T coverages and, hence, prevents direct photoemission. Timeresolved measurements enable to distinguish between intermediate states excited with 3.11 12


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or 4.22 eV.

Figure 5: (a) 2PPE intensity maps as a function of the final state energy and the pumpprobe delay (hν1 = 3.11 eV and hν2 = 4.22 eV) for α-6T coverages of 1, 2, 3 and 10 ML on Au(111). The left ordinate refers to final state energies Efinal above the Fermi level EF . The right ordinate to intermediate state energies for probing with hν2 = 4.22 eV. (b) 2PPE energy distribution curves as a function of final state energy at zero pump-probe delay taken from the maps shown in (a). (c) Single-color 2PPE intensity as a function of initial state energy measured with the UV pulses (hν2 = 4.22 eV) for coverages of 1, 3, and 10 ML. These spectra were assembled from two subsequent measurements with different energy ranges due to experimental restrictions of the measurable energy window. The pairs of spectra are normalized and stitched together at -1.4 eV. Figure 5(a) shows a series of intensity maps, where the 2PPE signal is plotted as a function of pump-probe delay and final state energy Efinal − EF for the four different coverages. For 13



positive delays, the sample is excited with the 3.11 eV pulse and probed with 4.22 eV; for negative delays, the order of pump and probe pulses is reversed. The color scale is defined from low background intensity (blue) to high intensity (white). All maps are normalized to the photon flux and plotted with identical color scale. Figure 5(b) shows the corresponding 2PPE energy-distribution curves at zero pump-probe delay. Sample work-function and peak positions can be extracted with a precision of ±0.05 eV (if not stated differently), which mainly reflects the width of the low-energy cut-off. Sample work-function The low-energy cut-off of the 2PPE spectra plotted on the final state energy-scale in Figs. 5(a) and (b) corresponds directly to the sample work-function Φ. With increasing α-6T coverage, we observe a decrease in the overall work function from 4.71 eV at 1 ML to 4.54 eV at 10 ML. Thereby, the largest drop in work function appears during adsorption of the first α-6T layer on the clean Au(111) substrate (ΦAu = 5.40 eV). This observation agrees with literature 8 and the PEEM results presented in Fig. 3. Electronic states of the wetting layer For 1 and 2 ML α-6T coverage, the overall shape of the 2PPE spectra looks similar (compare red and green electron-distribution curves in Fig. 5(b)). The dominant peak in the 2PPE spectrum appears at a final state energy of 7.06 eV. The peak intensity decays with a short tail towards negative delay (cf. Fig. 5(a)). Therefore, the 2PPE signal is related to a short-lived intermediate state at E1 − EF = Efinal − EF − hν1 = 3.95 eV, excited with hν2 = 4.22 eV and probed with 3.11 eV. The low-intensity peaks at 5.36 and 6.55 eV are centered at zero pump-probe delay and are therefore assigned to occupied initial states, ionized by direct two-photon absorption. The peak at 5.36 eV corresponds to a maximum in the occupied density of states of the gold d-bands at E − EF = −1.97 eV. A 2PPE spectrum of the clean Au(111) surface may 14


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be found in the supplementary material (SOM). The peak at 6.55 eV originates from an occupied molecular state at E − EF = −0.78 eV as further discussed below. Another peak in the spectrum of the α-6T monolayer is observed at a final state energy of 5.60 eV. Its peak maximum is shifted towards positive pump-probe delay. Therefore, it originates from a state at an energy of E − EF = 1.38 eV (cf. right ordinate of Fig. 5(a)). Electronic states of the crystallites Exceeding the wetting layer coverage, the tr-2PPE spectra change significantly, as exemplarily shown in Figs. 5(a) and (b) for nominal α-6T coverages of 3 and 10 ML. The spectra are now dominated by two additional broad peaks in the range from 5 to 6.3 eV final state energy. The time-resolved measurements reveal that these peaks originate from intermediate states excited with 3.11 eV, since their intensities exhibit pronounced tails towards positive delay (cf. Fig. 5(a)). The lifetime of the lower excited state increases with coverage. In addition, between 3 and 10 ML, its energetic position E − EF shifts from 1.28 to 1.16 eV. The state at E − EF = 1.8 eV appears as a clear peak for 3 ML α-6T coverage, while it only forms a shoulder in the spectrum of the 10 ML film. Its intensity decays with a lifetime of below 200 fs. Finally, in Fig. 5(c) we show monochromatic 2PPE spectra of α-6T films with nominal thicknesses of 1, 3, and 10 ML. In monochromatic 2PPE pump and probe photon stem from the same laser pulse having here a photon energy of 4.22 eV. The spectra exhibit two peaks originating from the gold d-bands at initial state energies of 2.1 and 2.6 eV below EF . Additionally, we observe a weak peak originating from an occupied initial state around E − EF ∼ −1.4 eV in the 3 ML case, which does not show up for 1 ML thick films. Its peak width increases with α-6T coverage.

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Discussion The Morphology of α-6T Films on Au(111) As shown in Fig. 2, PEEM images for a coverage of 9 ML reveal a clear brightness variation indicating the coexistence of sample areas covered with two different species. The bright spots are interpreted as α-6T crystallites with diameters in the order of 1 µm. The gray areas are attributed to a wetting layer. The total coverage of the wetting layer is deduced from the transients of the electron yield averaged over the entire field of view and its standard deviation. It is found to be 2.3 ML. The TDS results suggest an almost identical amount of molecules in the first and in the second densely packed layer of α-6T absorbed on Au(111). At a first glance, the appearance of the dip in the NSD of the PEEM measurements at a nominal coverage of 2.3 ML seems to contradict this interpretation, but one has to consider that the MEY measured with PEEM during film growth is a transient property. This means that kinetic effects can lead to a deviation from a perfect layer-by-layer growth and in particular the healing of defects (vacancies) in the first layer upon deposition of the second layer. Still we expect that the definition of the ML coverage as determined with PEEM and TDS coincide within 10 %. The Stranski-Krastanov type of growth at coverages above 2 ML can be understood from the fact that the relative orientation of neighboring molecules in the well-ordered monolayer strongly deviates from that in the α-6T bulk crystal structure. 17 While for the second layer, apparently it is still energetically favorable to adapt to the monolayer structure, a bulk-like structure is preferred for thicker multilayers. The formation of crystallites reduces thereby the interface area between the two incompatible structures. A plausible explanation for the maximum in the local electron yield around 0.5 ML is the combination of a signal increase for lowered work-function and a signal decrease due to dampening of the photoemission from the gold substrate, especially from the gold Shockley surface state (SSS), by the α-6T adlayer. While the dampening is expected to scale nearly linearly with coverage in the submonolayer 16


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regime, the change in work function does not, as shown by Blumenfeld et al. 18–20 For coverages exceeding the wetting layer, the average surface area covered by crystallites can be deduced from the evaluation of the histogram of the local electron yield. For the 9 ML film we find a fraction of 60 % of the surface covered by crystallites, resulting in an average height of the crystallites of 12 layers above the wetting layer. Based on our experience with α-6T deposition on silver surfaces, 9,11,21–23 we can assume that first a 2D gas, in which the molecules are highly mobile, is formed on top of the 2nd layer. Then, the 3D nucleation sets in at defects and finally at homogeneous nuclei. The mean electron yield depicted in Fig. 3 changes its slope exactly at the second minimum of the normalized standard deviation, identified as the completion of the wetting layer. Whereas the MEY increases linearly during the deposition of the second layer, the slope at higher coverage decreases continuously. This is an indication for 3D growth because the basal plane and the thickness of the crystallites increase sublinearly with the amount of deposited material. In addition, Ostwald ripening influences the size distribution of the crystallites. Summarizing, the PEEM results suggest that there exist at least three different condensed phases of α-6T on Au(111): 1.) the first layer in contact with the substrate, 2.) the second layer completing the wetting layer, 3.) molecules in 3D crystallites on top of the wetting layer.

The detailed knowledge of the film morphology turns out to be crucial for the interpretation of the coverage-dependent 2PPE experiments as it allows us to attribute distinct spectral signatures of the electronic structure to the wetting layer or the multilayer crystallites of α-6T on Au(111) (cf. Figs. 5 and 6).

Relation Between Geometrical and Electronic Structure The electronic structure of α-6T on Au(111) was previously analyzed for coverages of up to several monolayers by Varene et al. 8 In the present work, we connect the electronic structure 17



Figure 6: Energy diagram comprising occupied and unoccupied molecular states as well as excitonic states observed in the electronic structure of α-6T molecules through 2PPE studies of different α-6T coverages on Au(111). For comparison the electronic structure of the clean Au(111) surfaces is given at the left part of the figure. to the film morphology which leads in part to new assignments of the observed states. Figure 6 gives an overview of the electronic structure as a function of α-6T coverage. In the following, the specific electronic states will be discused in detail. Image-Potential or Interface State The most prominent peak in the 2PPE spectra for 1 and 2 ML of α-6T stems from the intermediate state at E − EF = 3.95 eV, labeled IS. Varene et al. 8 assigned this state to an image-potential state on the basis of its binding energy with respect to the vacuum level Evac of the first α-6T layer and its free-electron like dispersion parallel to the surface. For an imagepotential state located in front of the molecular layer, one would expect its binding energy and lifetime to be very sensitive to an increase of the layer thickness because the screening of the excess charge and the wave-function overlap with states in the metal substrate change. As such, the peak amplitude of an image potential state at the monolayer/vacuum interface should vanish upon completion of the second layer. Indeed, we observe a significant drop in the amplitude of the IS with increasing coverage, while energetic position and lifetime remain constant. However, the peak persists in the 2PPE spectra up to coverages > 10 ML with 18


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few percent of its intensity on the 1 ML film. Such a behavior would require that the overall structure of the wetting layer comprises areas of the bare monolayer in spite of formation of the crystallites on top of the wetting layer. Considering the homogeneous distribution of the electron yield observed in PEEM of the wetting layer, these monolayer areas have to be sufficiently small that they cannot be resolved in our PEEM measurements and sufficiently large to support the formation of an image potential state delocalized parallel to the surface. Alternatively, the IS could have the character of an interface state that is confined to the molecule-metal interface and thus insensitive to adsorption of additional α-6T on top. Such interface states delocalized parallel to the surface have been proposed by Bronner et al. for azobenzene derivatives on Au(111). 24 For an interface state, the peak intensity decreases with coverage due to dampening by the adlayers which the photoelectrons have to penetrate. This is fully in line with our experimental findings. In both cases, the IS can be probed even for multilayer coverages due to the bimodal film morphology, which consists of areas covered either with crystallites on top of the wetting layer or with just the wetting layer (cf. the example of a 30 ML film in the SOM). Furthermore, we cannot exclude that a signal contribution of the occupied Shockley surface state (SSS) is hidden beneath the peak of the IS. For the photon energies used in the experiment, the SSS of the clean Au(111) surface would appear in the 2PPE spectrum at a final state energy of 6.93 eV, which nearly coincides with the IS peak. Upon adsorption of an organic layer, the SSS is expected to shift upwards in energy due to hybridization. 25,26 Highest occupied molecular orbital For all investigated coverages between 1 and 10 ML we find a peak originating from an occupied state at E −EF = −0.78 eV. This peak is assigned to the highest occupied molecular orbital (HOMO) of α-6T molecules in the first layer. 8,27 While only a weak shoulder is observed in the monochromatic 2PPE spectra of Fig. 5(c), the HOMO appears as a clear peak in the time-resolved and bichromatic 2PPE measurements in Figs. 5(a) and (b), respectively. 19



To identify the HOMO of α-6T molecules not directly interacting with the metal substrate, we concentrate first on the monochromatic 2PPE spectra measured with a photon energy of 4.22 eV shown in Fig. 5(c). This photon energy is sufficiently separated from the S1 transition preventing excitation of the dominant Frenkel exciton. For 3 ML of α-6T on Au(111), the peak maximum is at E − EF = −1.4 eV. We assign this broad peak to a distribution of HOMO levels belonging to molecules within small crystallites on top of the wetting layer (labeled 3D HOMO). When increasing the coverage from 3 to 10 ML, the peak shifts to lower energies and its width and amplitude increase. Ultraviolet photoemission spectroscopy of multilayer α-6T films on various metal substrates report binding energies in the range of 1.6 to 1.8 eV, 28,29 well in agreement with our results. The presence of crystallites with different thicknesses is expected to increase the number of different local environments for the single molecule. Hence, it enlarges the variety of the energetic positions of the molecular orbitals and thereby the peak width. The assignment to an initial state is further corroborated by the photon-energy dependence of the monochromatic 2PPE shown in the SOM, Fig. S3. We explain the energetic shift of the HOMO from −0.78 eV in the wetting layer to −1.4 eV and lower in the crystallites by the different screening of the positively charged photoemission final-state. For molecules close to the substrate, the screening of the photohole is more efficient than for those close to the top of the crystallites. Comparable changes in binding energy of occupied states depending on the distance between molecule and metal surface were also observed in photoemission studies of different molecular layers at metal surfaces. 30,31 Frenkel Exciton For coverages exceeding the wetting layer, a signal related to an intra-molecular excitation within the organic thin film dominates the time-resolved 2PPE spectra in Fig. 5(a). We agree with Varene et al. that the long-lived part corresponds to the lowest Frenkel exciton (FE) excited by 3.11 eV photons. 8 The FE appears at an energy E − EF = (1.28 ± 0.1) eV in the case of the 3 ML film and at 1.16 eV in the 10 ML film (see right ordinate in Fig. 5(a)). The 20


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population of the state decays multi-exponentially with the longest decay time increasing from about 2.0 ps to more than 20 ps for increasing coverage from 3 to 10 ML. In this coverage regime, relaxation channels involving mutual charge and energy transfer between molecular layer and metal substrate dominate over photoluminescence. Since the Frenkel exciton is an electron-hole pair with the hole in the HOMO of α-6T molecules bound in crystallites, we expect to find a signature of the HOMO also in the time-resolved 2PPE spectra. Indeed, the short-lived higher energetic shoulder of the FE peak appears exactly at the final state energy where one would expect the 3D HOMO. To visualize this observation, we plotted the correlated signal for a coverage of 10 ML α-6T as a function of initial state energy in Fig. S4 of the SOM. Already in the case of the 2 ML thick film, we observe an increase of the background signal in the energy range, where the HOMO of the first layer appears. This might be related to the HOMO of molecules in the second layer or represent an onset of the 3D HOMO distribution. In the time-resolved measurements shown in Fig. 5(a), the peak maximum of the 3D HOMO is clearly shifted towards positive delay, indicating that we do not only observe the initial state distribution in this energy range, but that the excitation couples to intermediate states as well. These most likely are related to vibrationally excited Frenkel excitons. 9,10 In this case, the higher energetic shoulder is directly connected to the FE peak. Lowest unoccupied molecular orbital Whereas for coverages up to the completion of the wetting layer the Frenkel exciton is not observed, there exists an unoccupied electronic state at E − EF = 1.38 eV. Since it does not show up on the clean Au(111) surface, it is attributed to a molecular state. The amplitude of the corresponding peak in the spectra reduces with coverage in the same way as that of the d-band peak. Thus this state is part of the electronic structure of the first α-6T monolayer. Due to its short lifetime and energetic position we assign this state to the lowest unoccupied molecular orbital of α-6T molecules in the first layer occupied via charge 21



transfer from the metal substrate. Note that the LUMO in the monolayer is shifted towards lower energies compared to the LUMO in the crystallites because of screening of the excess charge. 31 Considering the energetic position of the HOMO, we find a transport gap of 2.16 eV for α-6T molecules in direct contact to the Au(111) surface. In the crystallites, the LUMO is not resolved due to the strong 2PPE signal of excitonic states in the same energy range.

Conclusion In this work, we combined PEEM, TDS, and tr-2PPE data to relate the electronic structure to the morphology of thin α-6T films on Au(111). We observed the formation of a bilayer thick wetting layer. Coverages higher than 4 ML clearly show the formation of µm-sized crystallites on top of this wetting layer. The co-existence of a 2D-wetting layer and 3D crystallites is corroborated by 2PPE spectroscopy, showing in parallel electronic states characteristic for the wetting layer and the crystallites. Only the 3D crystallites support the formation of an S1 Frenkel exciton with picosecond lifetimes. With increasing coverage, it shifts towards lower energies in a similar way as the HOMO of the α-6T molecules. When considering the electronic structure at the interface between an organic semiconductor and a metal contact, significant differences between the molecular bulk material and the layers directly interacting with the metal have to be taken into account. In view of the Frenkel exciton, the wetting layer serves as a spacer layer that decouples the intra-molecular excitations from the metal substrate. Multi-morphological growth has been reported for different types of organic molecules. Its influence on the occupied part of the electronic structure at the organic/inorganic interface has been pointed out for several systems. 33–36 We extend this discussion to the unoccupied part of the electronic structure including two-particle states as excitons, which is of particular interest for optoelectronic applications.

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Acknowledgement This work was supported by the Austrian Science Fund FWF under project numbers P24528N20 and P 28216-N36, by the Helmholtz Virtual Institute 419, and by the Freie Universität Berlin through the focus area Nanoscale Functional Materials and Systems.

Supporting Information Available • Bronsch_JPCC2018_SOM: Supplementary Material providing details on thermal desorption spectroscopy and additional 2PPE spectra of Au(111) and α-6T/Au(111). This material is available free of charge via the Internet at http://pubs.acs.org/.

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