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Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole-Phonon Coupling Yang He, Fabio Bussolotti, Qian Xin, Jinpeng Yang, Satoshi Kera, Nobuo Ueno, and Steffen Duhm J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04848 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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The Journal of Physical Chemistry

Transient Monolayer Structure of Rubrene on Graphite: Impact on Hole-Phonon Coupling Yang He1, Fabio Bussolotti2,#, Qian Xin3,#, Jinpeng Yang4,#, Satoshi Kera2,#,*, Nobuo Ueno5 and Steffen Duhm1,#,* 1

Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, People’s Republic of China 2 Department of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan 3 School of Physics, Shandong University, Jinan 250100, People’s Republic of China 4 College of Physical Science and Technology, Yangzhou University, Jiangsu 225009, People’s Republic of China 5 Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan #

Former address: Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan *Corresponding authors: E-mail: [email protected], phone: +81-564-557413(S. Kera); E-mail: [email protected], phone: +86-512-6588-0371 (S. Duhm) Abstract Charge transport in molecular thin films is often dominated by incoherent hopping processes and charge-carrier phonon coupling plays a major role in defining mobilities. Our high resolution angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) study reveals the influence of molecular configuration and packing on the hole-phonon coupling in vacuum-sublimed thin films of rubrene on graphite and allows determining charge reorganization energies. In the contact layer to the substrate rubrene is well-ordered with, for low coverages, the tetracene backbone being almost parallel to the graphite surface forming a loose-packed monolayer. Increasing the coverage leads to an orientational transition and a more tilted orientation of rubrene in a close-packed monolayer. This transition results in dramatic changes of spectral features in ARUPS including the photoelectron angular distribution of the highest occupied molecular orbital derived intensity. The charge reorganization energy, however, only changes slightly. The transient monolayer structure of rubrene on graphite allows thus to demonstrate that hole-phonon coupling in organic thin films does not depend very critically on the packing structure.

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Introduction Low charge-carrier mobilities are often limiting the performance of organic (opto-)electronic devices as charge transport in organic semiconductor thin films is often dominated by incoherent hopping processes1-4. Hopping mobilities depend critically on the charge-carrier phonon coupling5-8, which determines the charge reorganization energy λ. In most molecular thin films intermolecular interactions are weak and λ is consequently dominated by intramolecular vibrations and is often taken from gas-phase calculations5,9-11 or gas-phase ultraviolet photoelectron spectroscopy (UPS) data9,12-14. However, the relevant quantities for device application are the solid state values, which depend on the molecular surrounding, i.e. the interaction with neighboring molecules and/or a substrate7,15. Due to the similarities of ionization (followed by neutralization) during localized hopping transport and photoionization, UPS is the method of choice to measure charge reorganization energies of organic thin films by an analysis of the vibrational fine structure of the highest occupied molecular orbital (HOMO)-derived peak7,8,15-18. The relevant vibrational energies (hνi) of a molecule are often centered closely and in good approximation the analysis can be done by a single mode fit with one main vibrational energy (hν)19. Moreover, in most molecular thin films the relaxation energies for ionization and neutralization of a molecule are rather similar and the charge reorganization energy is thus simply given by λ=2Shν with S being the Huang-Rhys factor, which is determined by the relative intensity contribution to the vibrational progressions7,13. In single crystals field effect transistors 5,6,11,12-tetraphenyltetracene (rubrene) shows exceptionally high hole mobilities of up to 40 cm2/Vs20 and almost trap-free transport21. Moreover, rubrene shows high singlet fission efficiencies in both, single crystals22,23 and amorphous thin films24. Rubrene became thus a model system to study intermolecular interactions and charge transport properties of molecular solid states10,20,21,25-31. The outstanding charge transport properties might be related to the peculiar molecular structure of rubrene with the frontier orbitals mainly located on the tetracene backbone and the electronically inactive phenyl side groups28,32-34. In single crystals this shape leads to structure35 which allows overlap of molecular wave-functions10,29 leading to band dispersion with large HOMO band widths of ~0.4 eV, which have been experimentally determined by UPS27,36. Evaporated rubrene thin films, however, are often amorphous37-40 and mobilities low. This obstacle for device performance can be overcome by substrate pre-patterning41 and recently also in evaporated rubrene thin film hole mobilities of up to 11.6 cm2/Vs have been measured25. In solid states rubrene exhibits two conformations: In single crystals rubrene has a planar tetracene backbone35, whereas in the gas phase and in amorphous thin films the energetically more favorable29 conformation with a twisted backbone prevails10,38. In vacuum-sublimed monolayers on inert surfaces like Bi(001) or highly ordered pyrolytic graphite (HOPG) the two conformations can coexist26,42. In a 2


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previous study of monolayer rubrene on HOPG we showed that the main vibrational energy (determined in a single mode analysis of the vibrational fine structure of the UPS HOMO-derived peak) of the twisted conformation is slightly larger than that of the planar conformation26. The thin film structure and the conformation of rubrene on various substrates depend critically on the film thickness37,43-45. Thus, in order to cover a thickness regime which is more relevant for device application, we explore the hole-phonon coupling beyond monolayer coverage in our combined angle-resolved UPS and metastable atom electron spectroscopy (MAES) study of vacuum-sublimed rubrene on HOPG Experimental section Rubrene (two times purified in high-vacuum) thin films were prepared by vacuum sublimation on clean HOPG surfaces using resistively heated quartz crucibles with deposition rates of about 0.25 Å/min as controlled by a quartz crystal microbalance. HOPG (ZYA grade) was cleaved in air just before loading into the preparation chamber and cleaned by in situ heating at 420°C for 11h. ARUPS and MAES experiments were performed using a custom built apparatus equipped with a HeI (21.22 eV) UV-light-source, a He* source and a hemispherical electron energy analyzer (Scienta R3000). The interconnected sample preparation chamber (base pressure 2*10-8Pa) and analysis chamber (base pressure 4*10-8Pa) allowed sample transfer without breaking ultrahigh vacuum conditions. For MAES He* (23S; 19.82 eV) was used for excitation with an incident angle of 45° in normal emission with the sample biased at -3V. For ARUPS the angle between the incident light and the sample was fixed to 65°. The spectra were measured at photoelectron emission angles ranging from 0° (normal emission) to 50°. A sketch of the experimental geometry can be found in Ref.26. The spectra are angle integrated over ±10° perpendicular to the surface-normal/detection plane. The UPS energy resolution was set to 80 meV at room temperature (295 K) and to 30 meV at low temperature (35 K) as determined by the Fermi-edge-width of a Cu(111) sample. All preparation steps were performed at room temperature, measurements at room temperature and 35 K, respectively. Additional UPS spectra were acquired in normal emission at 140 K by an ultrahigh-sensitivity UPS apparatus with a hemispherical electron energy analyzer (MBS A-1) and monochromatic XeIα (hν = 8.44 eV) and HeIα radiation sources46. In this system the energy resolution was set to 30 meV. Fitting of the spectra was done by Winspec (developed at Namur University, Belgium) according to Ref. 26 with two individual series of Voigt-peak line shapes. The error for the vibrational energies is estimated to +/- 5 meV (for larger deviations no converging fitting results could be obtained). The spread in Huang-Rhys factor for different fitting approaches was +/0.05. Results Figure 1a shows UPS spectra of rubrene with increasing thickness on HOPG. The photoemission at around 1.2 eV binding energy (BE) is assigned to stem from the HOMO of rubrene. The photoemission intensity around 8 eV mainly stems from 3



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molecular orbitals located at the phenyl rings of rubrene26. All molecular features exhibit dramatic changes in peak position and shape with increasing film thickness, which is in qualitative agreement with recently published rubrene/HOPG UPS data47. For example, the sharp HOMO-derived peak shifts slightly to higher BE by increasing the coverage from 3 Å to 6 Å. Further increasing the coverage makes the peak broad and featureless. Such a behavior points to a transition from well-ordered (up to 6 Å coverage) to amorphous (beyond 6 Å coverage) growth. The substrate derived peak at 13.6 eV BE which is assigned to a final state structure related to the HOPG σ* band48,49, reaches a maximum for a coverage of 3 Å. In MAES the sample is excited with metastable He* atoms, which cannot penetrate the sample surface50. Thus MAES is ideally suited to check the formation of wetting layers. In the MAES spectra of rubrene on HOPG (Figure 1b) the σ*-peak (here centered at 12.2 eV BE) 1, vanishes for 3 Å coverage. This coverage corresponds thus to a totally closed monolayer (ML) of rubrene on HOPG, as already was shown in Ref.26. The vacuum level (as determined by the low kinetic cutoff of secondary electrons, spectra not shown) is at 4.52 eV above the Fermi-level for clean HOPG and stays constant at 4.47 eV (within +/- 3 meV) for all rubrene coverages. Figure 2 compares the HOMO-derived peaks of different coverages (nominally 3 Å, 4.5 Å and 6 Å) at 35 K. It becomes clear that not only the main feature is shifting to higher BE, but that also the photoemission-intensities for the two emission angles (0° and 45°) are changing heavily by going from a coverage of 3 Å to 6 Å. These changes can be attributed to a change of the molecular conformation and/or orientation. For reasons explained in detail below we conclude that 3 Å coverage corresponds to a transient structure, the “loose-packed monolayer” and 6 Å coverage also represents a monolayer, the “close-packed monolayer”, with the 4.5 Å coverage being an intermediate case with contributions from both, loose- and close-packed ML. For all coverages the HOMO-derived peaks exhibit a pronounced vibrational fine structure. For the loose-packed monolayer (3 Å coverage) a detailed analysis was done in Ref.26. An analogous fitting approach (single mode fit) was applied to the HOMO-derived peak of the close-packed ML (6 Å coverage), the results are displayed in Figure 3. Like for the loose-packed ML, the peak can be fitted with two independent vibrational series, which are attributed to rubrene in the twisted and the planar conformation. Fitting was done in loops, first the full width at half maximum (FWHM) 1

As the σ*-peak originates in an unoccupied orbital it has to be first get occupied by secondary electrons (originating from HOPG or the adsorbate) before being detectable in UPS or MAES. Thus, the kinetic energy of the σ*-derived electrons does not depend on the excitation energy (21.2 eV for UPS and 19.8 eV for MAES) and the σ*-peak occurs at different binding energy values for spectra plotted with respect to the Fermi-level. Usually, more secondary electrons are emitted from organic layers than from the HOPG. Thus, in UPS the σ*-peak often has a maximum for monolayer coverage. 4


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and the Gaussian contribution was determined to 85 meV and 95%, respectively, and then fixed for all peaks. The energetic positions of the 0-0 transition of each series and the intensities of all peaks were fitting parameters. The spacing (hν) of the peaks was set to 159 meV for the twisted series (mainly determined by the spacing of the 0-0 and the 0-1 transitions) and 156 meV for the planar series. The Huang-Rhys factor S (determined by the intensity ration of 0-0 and 0-1 transitions) for the twisted conformation is 0.52 and 0.61 for the planar conformation, which gives charge reorganization energies of 165 meV and 190 meV, respectively. Both of the values are slightly larger than the charge reorganization energies in the loose-packed monolayer26. In addition to these two series an additional peak (labeled “D” in Figure 3c) was necessary to model the experimental data. In order to get insight in the orientation of rubrene in the close-packed monolayer the photoelectron angular distribution (PAD)51-53, i.e., the photoemission intensity as function of emission angle, was recorded. Figure 3d compares the intensities of the 0-0 transition peak of the close-packed ML in different emission angles with these of the loose-packed ML and the additional peak D. The PAD of the close-packed ML is distinctively different from the PAD of the loose-packed ML, which clearly demonstrate a change in molecular orientation with respect to the substrate. The sharp distribution of photoelectron intensity with the maximum at an emission angle of 40° for the loose-packed ML is typical for π-conjugated flat-lying molecules19,54. As the HOMO of rubrene is mainly located at the tetracene backbone it was concluded that in the loose-packed ML the molecular long and short axes of the tetracene backbone are almost parallel to the surface26. A flat-lying orientation of rubrene in the contact layer with HOPG and graphene was also recently shown by means of scanning tunneling microscopy (STM)45,47. For the close-packed ML the PAD is rather broad with no clear maximum. Consequently, for this coverage the molecules adopt a somehow tilted orientation. The mean free path of photoelectrons with a kinetic energy of around 15 eV in organic materials is in the range of 5 to 10 Å55 and, moreover, for a nominal coverage of 6 Å the substrate derived σ* peak is clearly still visible (Figure 1a). The wetting layer (3 Å nominal coverage) would thus not get totally masked in the UPS spectra for nominal bilayer (6 Å) coverage. However, no traces from the loose-packed ML PAD can be found in the close-packed ML PAD. Thus, the flat-lying layer must be a transient structure, i.e., the loose-packed ML (3 Å nominal coverage) is changing to a close-packed ML (6 Å nominal coverage) of tilted molecules. A more tilted orientation was also shown by STM for higher coverages and assigned to rubrene in a second layer growing on top of the flat lying contact layer. However, in STM it is hard to distinguish whether a first or second layer is imaged56,57, thus the STM results are not in apparent conflict with the growth scenario concluded from ARUPS. As the vacuum level stays constant for all coverages, the small shift (0.08 eV) in binding energy of the first ionization peaks of twisted and planar rubrene between loose-packed and close-packed ML translates into an increase in the ionization 5



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energy (IE). For related molecules like anthracene16, tetraseleno-tetracene58 or pentacene59 the IE is decreasing by going from lying to tilted/standing orientations due to a collective impact of molecular surface dipoles on the potential energy (i.e., the vacuum level)60. However, although the frontier molecular orbitals of rubrene are mainly located at the tetracene backbone, the phenyl-side groups also have intrinsic surface dipoles32. The potential energy of the molecular thin films is the result of the impact of all molecular surface dipoles60. As in rubrene the molecular dipole moments of backbone and phenyl side-wings are almost perpendicular to each other, a change from lying to tilted can lead to the observed increase in ionization energy. The PAD of the so far unassigned peak D is different to that of both, the loose-packed and the close-packed ML (Figure 3d). The energetic distance to the 0-0 transition of twisted rubrene is 107 meV, thus it does not fit in one of the two vibrational series. The origin of this peak could thus be defects with another orientation and/or conformation of rubrene on the HOPG surface. However, also peak D has a narrow FWHM (90 meV), which would be rather unusual for a defect-derived peak. Moreover, several individual samples (prepared in two different experimental setups) showed virtually the same relative intensity and BE position of peak D. Another possible origin of peak D is a quasi-particle state like a fully relaxed polaron2,52,61,62. In general, HOPG with its low density of states close to the Fermi-level can lead to long polaron lifetimes. Moreover, two-photon photoemission of rubrene on HOPG revealed a rather special coupling, including formation of a superatom molecular orbital like state30,47,63. For a quasi-particle state a dependence of the lifetime and cross section on the excitation energy could be expected. Thus, we performed additional UPS measurements with different photon energy (8.44 eV of XeIα in contrast to 21.22 eV of HeIα), the results are shown in Figure 4. The spectra are virtually the same in both, intensity and peak position of peak D and the HOMO-derived peak. The higher background at the higher binding energy side in the XeI spectrum is due to the larger contribution of secondary electrons for spectra measured with smaller excitation energy64. The negligible differences in HeI and XeI spectra point to another origin of peak D than a quasi-particle state. However, it also could be that the differences are too subtle to be evidenced within experimental limitations. Thus, the origin of peak D remains unclear and subject of further investigations. Discussion Transitions from flat lying monolayers to tilted multilayers are often observed for vacuum-sublimed COMs on metal surfaces65,66, and also transient monolayers are sometimes observed in organic thin film growth67-70. However, the growth mode of rubrene is remarkable: In both, the loose-packed and the close-packed ML, the HOMO-derived UPS-peaks are very sharp and vibrational progressions can be observed up to the 4th order. This points to a high degree of order in both layers, although the twisted and the planar conformation of rubrene are coexisting. Moreover, even this highly ordered thin film cannot act as template layer for ordered 6


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multilayer growth, as for coverages beyond monolayer the spectra become broad and featureless. The vibrational properties, i.e., the Huang-Rhys factors, the vibrational energies and the charge reorganization energy, of rubrene in the loose-packed26 and the close-packed ML on HOPG are summarized in Table 1. The reorganization energies are increasing from loose-packed ML to close-packed ML. In the close-packed ML molecules are stacked in a tilted orientation, while in the loose-packed ML molecules lie flat on the substrate. Thus, the distance between adjacent molecules is larger for loose-packed ML and consequently the intermolecular interaction is weaker, which accounts for a smaller reorganization energy. For both monolayer coverages the vibrational energies and the PADs for twisted and planar conformations are very similar. Only the Huang-Rhys factors in the close-packed ML are significantly different. However, especially for the planar conformation, which has a smaller contribution to the close-packed ML spectra, the Huang-Rhys factor depends critically on the fitting procedure (e.g., the FWHM) and the given data does not thus not allow drawing solid conclusions about this difference in the Huang-Rhys factor. Overall, for rubrene in the two very different monolayer structures on graphite the hole-phonon coupling is rather similar. However, for charge transport also the intermolecular transfer integral t is important5,52. Transfer integrals of organic thin films can be experimentally determined by UPS band dispersion measurements52,71,72. However, due to the similarities of the relevant energies, i.e., the vibrational energies and the bandwidth, which are both often in the range of a few hundred meV, it is hard to measure both for the very same sample. Our measurements show that for rubrene the actual thin film structure plays a minor role for the charge reorganization energy. The transfer integral, however, obviously depends dramatically on the structure. The exact structures of the loose-packed and the close-packed ML are unknown. However, for flat lying molecules in-plane transfer integrals are notoriously low73, but for the stacked orientation in the close-packed ML the in-plane transfer integrals are expected to be much larger. Consequently, the change from loose-packed to close-packed should increase the mobility dramatically, although the charge reorganization energy is slightly increasing. Conclusion Our study reveals the initial stages of rubrene growth on graphite and correlates them with charge transport properties. For low coverages, a loose-packed monolayer of flat lying rubrene molecules is formed. Increasing the coverage leads to a transition to tilted molecules in a close-packed monolayer with twisted molecules dominating. The increased intermolecular interaction in this tilted structure leads to a small increase in the main vibrational energy and to an increase in the charge reorganization energy. Our study shows that the influence of orientation and conformation of rubrene thin films on the hole-phonon coupling is relatively small. This might be general for molecular semiconductors thin films and the transient 7



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monolayer structure of rubrene on graphite allows to demonstrate that the actual thin film structure has a minor impact on hole-phonon coupling. Acknowledgments We thank Prof. Haiming Zhang for fruitful discussions about rubrene thin film growth. Financial support from the Major State Basic Research Development Program of China (973 Program, Nos. 2013CB933500 and 2014CB932600), JSPS KAKENHI (Nos. 26248062 and 23360005), the Global-COE Program of MEXT (G03: Advanced School for Organic Electronics, Chiba University), an NSFC Research Fund for International Young Scientists (No. 11550110176), NSFC (No.61504119), the Jiangsu Province Technology Fund (Nos. BK20141188, BK20150458), a joint JSPS-NSFC project (No. 612111116) and the Collaborative Innovation Center of Suzhou Nano Science & Technology (NANO-CIC) is gratefully acknowledged

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Table 1. Measured values of the Huang – Rhys factor S, the main vibrational mode hν, and the charge reorganization energy λ for rubrene in the loose-packed monolayer (taken from Ref. 26 and the close-packed monolayer (this work) on graphite.

loose-packed ML close-packed ML

twisted planar twisted planar

S 0.58 0.62 0.52 0.61

hν (meV) 134 127 159 156

14


λ (meV) 155 157 165 189

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FIG. 1: (a) Room temperature UPS spectra of rubrene/HOPG with increasing film thickness (χ). The survey spectra were measured at photoelectron emission angles (θ) of 0° and the close-up spectra of the HOMO region with emission angles of 45°. (b) Thickness dependent metastable atom electron spectroscopy (MAES) spectra of rubrene/HOPG. All spectra are plotted w.r.t. the Fermi-level.

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FIG. 2: Comparison of the UPS HOMO peak of rubrene on HOPG measured at 35 K for different coverages: loose-packed monolayer (nominally 3 Å), intermediate layer (4.5 Å) and close-packed monolayer (6 Å). Spectra measured at emission angles (θ) of 45° and of 0°, respectively, are displayed.

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FIG. 3 Fits to the background subtracted UPS HOMO peaks of the close packed monolayer (nominal coverage: 6 Å) measured at 35 K for emission angles (θ) of (a) 0° and (b) 45°. (c) Same for the angle integrated spectrum (sum of spectra measured with 10° steps from θ of 0° to 50°). The transitions for rubrene in the twisted conformation (green) and in the planar conformation (blue) are marked. (d) Normalized photoelectron intensity of the 0-0 transition of the twisted (TW) and the planar (PL) conformation of the close packed monolayer and loose packed monolayer (taken from Ref. 26) rubrene on HOPG as a function of the photoelectron emission angle. 17



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FIG. 4 Normalized photoelectron spectra of close-packed monolayer rubrene on HOPG measured in normal emission at 140 K with XeIα and HeIα radiation, respectively. The difference in integral momentum space by the different photon energies is not corrected in the figure, but has only a minor influence on the spectral shape (not shown) and no influence on the binding energies of the peaks.

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Table of Content Figure

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(a)

(b)

(c)

(d)


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Transient Monolayer Structure of Rubrene on ... - ACS Publications

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