Tunable Interface Properties between Pentacene and Graphene on

Article pubs.acs.org/JPCC

Tunable Interface Properties between Pentacene and Graphene on the SiC Substrate Xianjie Liu,*,† Alexander Grüneis,‡ Danny Haberer,§ Alexander V. Fedorov,§ Oleg Vilkov,∥ Wlodek Strupinski,⊥ and Thomas Pichler‡ †

Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden Faculty of Physics, University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria § Leibniz IFW-Dresden, P.O. Box 270116, D-01171 Dresden, Germany ∥ St. Petersburg State University, St. Petersburg, 198504, Russia ⊥ Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland ‡

ABSTRACT: Understanding energy-level alignment and molecular growth characteristics of an organic semiconductor on the graphene surface is crucial for graphene-related device performance. Here we demonstrate that tunable interface properties and molecular orientation can be achieved by modifying graphene films on a SiC substrate with monolayer copper-hexadecafluorophthalocyanine (F16CuPc) molecules. On clean graphene, pentacene molecules form a tilted configuration even at very low coverage (one or two monolayers) rather than flat-lying as on the graphite surface. Pentacene molecules prefer to grow with a (022) plane parallel to the clean graphene surface. With increasing coverage, X-ray adsorption data indicate there is no obvious change of molecular stacking orientation. The corresponding hole injection barrier is about 0.7 eV. On the modified graphene where thin (one or two monolayers) F16CuPc molecules are flatlying on graphene, an almost perfect up-standing molecular stacking of pentacene film was formed on the modified surface. A low hole injection barrier of 0.3 eV was observed. Furthermore, the interface of dirty graphene upon pentacene was also discussed.

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INTRODUCTION The unique electronic and mechanical properties of graphene films1 have triggered intensive investigations related to their perspective applications. Among them, the feasibility of graphene as a transparent electrode material in organic electronic devices is particularly intriguing.2−6 Especially, a crystalline monolayer graphene adsorbs only 2.3% of light.7 Moreover, graphene also processes high chemical and thermal stability,8 high stretchability,9 and low contact resistance with organic materials,8,10 which offer tremendous advantages of graphene as a promising transparent conductor in organic electronic devices and even flexible graphene-based organic photoelectrical devices. Up to now, graphene-related organic devices based on transferred graphene films have been reported4,9,11 after the breakthrough of graphene synthesis on Cu metal surfaces12 with highly uniform, large-scale, and transferable films. However, due to a lack of information on the influence of the surface properties of graphene upon the growth characteristics of organic semiconductors, most works only focused on the intrinsic conductivity properties of the used graphene.4,9 Although there are some reports of organic semiconductor growth on reduced graphene oxide films,13 their surface properties are totally different from those of © 2013 American Chemical Society

pristine graphene. On the other hand, there have been intensive investigations of the molecular arrangement of organic semiconductors on highly ordered pyrolytic graphite (HOPG) or graphite, in which organic molecules with aromatic rings will interact with underlying HOPG by π−π interactions.14−18 However, it is difficult to achieve such quasiepitaxial growth mode in device fabrication. Furthermore, regarding the difference of graphene compared to graphite-like HOPG, the influence of the substrate under graphene on the molecular growth modes must be considered. Although there are some reports of organic semiconductors on large-area graphene grown on a SiC surface by scanning tunneling microscopy (STM),19,20 the film thickness needed in organic semiconductor devices is far beyond the reach of STM. The functionalities of organic electronic devices are often determined to a large extent by the energy-level alignment at the various interfaces between organic/organic and organic/ electrodes.21−23 When organic semiconductors are deposited on electrodes, the formation of interfacial dipole often occurs. Received: October 18, 2012 Revised: January 18, 2013 Published: January 24, 2013 3969

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As a result, a vacuum level alignment based on the Schottky− Mott model cannot be applied. Although several models for the energy-level alignment at organic interfaces have been developed,23−25 there is no universal one to apply for all interfaces. Energy-level diagrams often cannot be obtained merely by using the values of ionization potential (IP), electron affinity, and work function of the materials involved. Furthermore, molecular order/disorder at the interface plays another important rule in the context of energy-level alignment. The influence of molecular orientation on the value of IP will consequently affect charge carrier barriers.26 Therefore, understanding graphene interfacial electronic properties is a key issue to improve the graphene-related device performance.27 Because graphene cannot solely exist as a form of free-standing film, in a device the interface between organic materials and graphene cannot simply be taken into account as two materials themselves. The supporting material underneath graphene can strongly influence the electronic properties of graphene including the work function. Taking into account the effective interaction regime of the organic interface, monolayer graphene is much thinner than the thickness of the organic interface.28 In this case, the total interface properties of graphene cannot exclude the influence of the supporting material upon the electronic properties of organic materials deposited afterward, which leads to different interface properties compared to graphite. Therefore, the change of the size of the interface dipole, as well as the corresponding charge injection barrier and the molecular order/disorder at the interface, must be extensively examined to draw a complete picture of the graphene-related interface and to improve the graphene-based organic device performance. Here we present detailed photoemission and X-ray adsorption studies of interface properties between pentacene molecules and graphene and further modified with copperhexadecafluorophthalocyanine (F16CuPc). On clean graphene substrate, pentacene was grown with a tilt angle of about 29° relative to the surface independent upon the film thickness, in which a hole injection barrier of 0.7 eV was observed. Upon F16CuPc modification, the F16CuPc molecules are completely flat-lying on the graphene, while the pentacene molecules almost perfectly stand on the modified graphene surface. Correspondingly, a lower hole injection barrier of 0.3 eV was achieved. Such orientation is optimal for charge transport along the graphene surface. Furthermore, the pentacene growth characteristics on uncleaned graphene are also discussed.

step well above the threshold. The energy resolution was about 100 meV at photon energy close to the C K-edge. Thin films of pentacene (99.9% Syncom BV) with different thicknesses were prepared on graphene films by in situ vacuum evaporation in a preparation chamber (base pressure 1 × 10−9 mbar) with an evaporation rate of 1−2 Å/min from a Knudsentype organic evaporator. F16CuPc (Sigma-aldrich) was purified twice before use. The thickness of the respective molecule layer was estimated by monitoring the attenuation of the intensity of the core-level signals of the bottom layer (i.e., Si 2p) due to the organic overlayer deposition.29 High-quality uniform graphene was synthesized on an n-type doped 4H-SiC(0001) substrate by chemical vapor deposition,30 in which graphene can be grown on an insulating and conductive SiC substrate and the number of graphene layers and growth rate can be preciously controlled. Such growth can overcome the disadvantage of graphene growth on a metal surface that electronic application requires graphene on an insulating substrate. The quality of graphene was checked with transmission electron microscopy, Raman, and angle-resolved photoemission spectroscopy (ARPES).30 All graphene films were cleaned in vacuum by annealing at 500 °C for 24 h before organic material deposition.

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RESULTS AND DISCUSSIONS Pentacene on Monolayer Graphene. In the monolayer graphene on SiC, the first graphene layer, the so-called buffer layer, is covalently bonded to the SiC crystal and van der Waals bonded to the second graphene sheet, the so-called monolayer graphene, which is responsible for the graphene-type electronic spectrum observed experimentally.31 When organic semiconductors are deposited on the graphene, the formation of an interface dipole will manifest itself through a change in the corresponding work function. Studying the evolution of the valence band spectra with different thicknesses of the molecular films is the normal method to probe the interface properties. Figure 1 clearly describes the valence band feature of pentacene on graphene/SiC with increasing coverage and the corresponding low-kinetic secondary electron cutoff at the photon energy of 70 eV. At low pentacene coverage of 0.7 nm,

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EXPERIMENTAL SECTION Synchrotron experiments were carried out at BESSY RGBL (Germany) and Maxlab D1011 (Sweden) beamlines. All photoemission spectra were acquired in normal emission. The valence band spectrum was calibrated by measuring the Fermi level of a clean gold film. The work function was extracted from the position of the low-kinetic energy secondary electron cutoff (onset of photoemission) for each valence band spectrum by applying a negative bias to the sample. The X-ray absorption spectrum (NEXAFS) with different incident angles was collected by directly measuring the sample current (total electron yield) and using a multichannel plate electron detector (partial electron yield with a retard voltage of −100 V) simultaneously. The raw data have been corrected for the energy dependence of the incident X-ray beam and subsequently normalized to have the same absorption edge

Figure 1. Evolution of the valence band spectra and the corresponding secondary electron cutoff photoemission spectra of the pentacene films on clean monolayer graphene with an increase of coverage at the photon energy of 70 eV. 3970

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the spectral feature of pentacene is already visible. It can be clearly seen that, up to 2.5 nm pentacene, the frontier valence feature is fully developed. There is no discernible change in peak width of the pentacene highest occupied molecular orbital (HOMO) as a function of further increasing coverage nor in the position of the edge of the HOMO with respect to the Fermi edge. Also the secondary electron cutoff is kept unchanged. The hole injection barrier of the pentacene films is about 0.7 eV, which is very similar to the value of pentacene on graphite. At high pentacene coverage, the only slight change is the intensity of the HOMO and HOMO-1 peaks. The interface dipole between pentacene and graphene is about 0.1 eV extracted from their work function evaluation. Interestingly, when graphene films were transferred to the SiOx/Si substrate, the interface dipole is about 0.15 and 0.4 eV between pentacene film and the transferred monolayer graphene32 and multilayer graphene,33 respectively. A very small interface dipole less than 0.1 eV was observed between pentacene and HOPG.14 The different work function of graphene/SiC from HOPG can be attributed to the fact that the graphene on SiC is electron doped due to the substantial interaction between the graphene and SiC substrate underneath which has been observed in ARPES measurements.34 From the observed shift of Dirac point relative to the Fermi level, which is about 295 meV in the sample, a charged graphene single layer with 0.0078 e− per carbon atom can be extracted.20,30,35 On the other hand, such variation of the size of the interface dipole clearly indicates the influence of the underlying substrate close to the top interface. Therefore, by selecting different substrates, it is possible to tune the graphene work function, as well as the interface dipole and the hole injection barrier. Besides the height of the injection barrier, the molecular stacking and crystalline orientation of thin films is another key parameter for optimization of thin film devices. The molecular arrangement at the initial stage of the film formation is largely governed by the interplay of intermolecular and molecule− substrate interaction. The inertness of graphene can avoid strongly binding molecules at the interface, and the carbon lattice of the basal plane of graphene is almost identical to the carbon frame of the molecular aromatic rings which may favor an epitaxial order at the interface. Angle-dependent X-ray adsorption is a powerful tool to probe the electronic coupling and orientation order of organic molecules in the various films.36,37 Figure 2 demonstrates the NEXAFS spectra of pentacene film as a function of the film thickness at the incident angle 50° between the X-ray beam and the sample surface. For clean graphene, the absorption feature consists of two distinct peaks at photon energies of around 285.4 and 291.8 eV for π* and σ* resonances, respectively. In a pentacene film, characteristic signatures in NEXAFS spectra are distinct resonances at photon energies of 283−287 eV due to the excitation of C 1s electrons into closely spaced unoccupied π* orbitals as well as broad resonance at higher energies which are attributed to excitations into σ* orbitals. Upon 0.7 nm pentacene deposition, the pentacene adsorption feature is partially overlapping with the π* region of graphene, and the absorption feature of graphene (sharp peak at 291.8 eV) is still detectable. When pentacene film thickness reaches 2.5 nm, the absorption feature from graphene is completely suppressed. This indicated the pentacene is fully covering on the graphene. In contrast, for a pentacene deposition on graphite, even at 20 nm thick pentacene film, graphite feature is still clearly visible.18 Such a difference demonstrates that different growth processes of

Figure 2. Dependence of C K-edge NEXAFS spectra of pentacene films upon film thickness compared to the clean graphene spectrum at the incident angle of 50°. The labels A and B indicate π* absorption features of the pentacene films.

pentacene occur on graphene and graphite. A layer-by-layer formation of pentacene on graphene is more favorable as compared to layer-island growth of pentacene on HOPG. Further increasing pentacene coverage, a very similar NEXAFS spectrum was observed, suggesting that the film grows continuously and no change of molecular orientation. Theoretical analyses have shown that the intensity of the π* resonance in NEXAFS depends on the polarization of the incident synchrotron light relative to the ring of aromatic molecules.38 Therefore, the average tilt angle of molecules can be determined from NEXAFS measurements recorded for different angles of incidence of the incoming light. For substrates with 3-fold symmetry, this yields the expression38 ⎡ ⎤ 1 I(θ ) ∝ P ⎢1 + (3 cos2 θ − 1)(3 cos2 α − 1)⎥ ⎦ ⎣ 2 + (1 − P)sin 2 θ

where θ is the beam incident angle relative to the sample surface; P is the degree of the beam polarization; and α is the tilt angle of molecules to the surface. A quantitative analysis of the dichroism of the π* resonance of the pentacene film can yield the tilt angle of the molecules on the substrate. Figure 3 describes the change of the relative π* intensity of pentacene films at different incident angles for variable film thickness. Meanwhile, the theoretical analysis of the intensity dichroism is presented as a dashed line in the figure. At nominal thickness of 0.7 nm, the quantitative analysis yields an average tilt angle of 26° ± 5°, which reflects a rather recumbent orientation of the pentacene molecules. This orientation compares well with the molecular arrangement adopted in the preferentially formed (022) plane of pentacene which yields an average tilt angle of the aromatic ring plane of 28° relative to the surface plane.18,39 Considering the nominal thickness of 0.7 nm corresponds to about two pentacene molecular layers and assuming planar adsorption geometry in the very first monolayer and a tilted assembly in the following layer, an average molecular tilt of 56° would be expected. A transition of pentacene film on HOPG from the laying-down configuration at monolayer coverage to the standing-up one with a thickness increase has been observed,16 but in our case, with further increasing pentacene coverage, there is no change of the molecular stacking observed compared to the initial two layers. Since the surface of graphene 3971

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Figure 4. Valence band features of pentacene films on F16CuPcmodified graphene at different thicknesses and the corresponding secondary electron cutoff. The spectra of thin F16CuPc and clean graphene are also presented.

Figure 3. Dependence of the relative NEXAFS C 1s π* intensity of pentacene films at different thicknesses upon the X-ray incident angle. The dashed lines are the fitting curves to extract the pentacene orientation degree.

corresponding theoretical calculations,20,35 it is still hard to extract the exact amount of charge transfer in our macroprobe spectroscopic data. NEXAFS is a direct measure of the unoccupied states of F16CuPc molecules. By comparing the N K-edge NEXAFS spectra of the monolayer F16CuPc molecule (shown in the inset of Figure 5) with the published spectrum of

on SiC is very flat and large scale, one reasonable explanation could be that the molecules in the bottom layer of such multilayer films are actually pitched and remain no longer parallel to the graphene surface.18 The slightly different tilt angle compared to the (022) plane could be due to the substrate charge effect of SiC on the supported graphene film compared to HOPG with the all-carbon layer, and also the uncertainties in estimating the tilt angle resulted from the weak NEXAFS signals from the very thin pentacene film. With further pentacene deposition, the quantitative analysis of the corresponding dichroism of π* intensity yields an average tilt angle of 29.2°, 29°, and 28.5° for 2.5, 5, and 7 nm pentacene film, respectively. It clearly demonstrated that the film structure is robust, and molecular orientation does not depend on the thickness. Therefore, pentacene molecules are preferentially formed on the (022) plane on the graphene/SiC surface to stack a thick film. Pentacene on F16CuPc-Modified Monolayer Graphene. The application of graphene strongly depends on how to tune its electronic properties in a controlled manner. Dependence on the electron affinities of the dopants n- or ptype doped graphene can be reached. On the other hand, the interplay of intermolecular and molecule−substrate interaction strongly influences molecular arrangement, especially at the initial stage of the organic film formation. Therefore, doping will not only influence the interface dipole between graphene and the deposited organic molecules but also affect the interaction strength between the organic molecules and the graphene substrate through which an approach to control the molecular orientation and hence the film structure can be achieved. Figure 4 describes photoemission spectra at the valence band region of pentacene on F16CuPc-modified graphene, in which about 0.6 nm thick F16CuPc film (around monolayer coverage) was deposited, and clean graphene. One obvious change is the work function of the graphene from 4.46 to 4.6 eV by the modification of F16CuPc, which is an electron acceptor with high electron affinity. The resulting work function of the graphene is close to that of HOPG. Although there are reports of strong charge transfer from monolayer graphene to the F16CuPc molecule in local-probe STM experiments and the

Figure 5. Dependence of C 1s NEXAFS spectra of pentacene films on F16CuPc-modified graphene upon the incident angle. The inset illustrated the N 1s NEXAFS spectra at two different angles.

the lying F16CuPc molecules,40 the change of LUMO of the F16CuPc molecule due to the filling of the transferred charge is quite weak and less than 8%. Further ARPES measurements on monolayer F16CuPc-modified graphene will be a powerful tool to extract the exact amount of the charge transfer. Nevertheless, the charge transfer can increase the work function of graphene film through the formation of an interface dipole. Correspondingly, a change of graphene work function will reflect a variation of the interface dipole between pentacene and graphene. The measured work functions difference indicated the interface dipole is less than 0.05 eV. Considering that the total experimental energy resolution is around 0.1 eV, vacuum level alignment occurs between the pentacene film and the F16CuPc-modified graphene, contrary to pentacene on the bare graphene, where the interface dipole is around 0.15 eV. 3972

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Figure 6. Energy-level diagram of pentacene film on F16CuPc-modified monolayer graphene/SiC (left panel) and on clean monolayer graphene/SiC (right panel). All energy values are expressed in eV.

barrier of the pentacene film, such results will be beneficial for graphene-related device applications. Consequently, the energy-level alignment of pentacene on F16CuPc-modified graphene and clean graphene can be summarized in Figure 6. First, for simplicity, graphene/SiC was considered as metallic characteristics by the position of Fermi level. Second, due to the substrate background, the frontier valence region maximum (HOMO) of F16CuPc can only be roughly estimated at around 1.3 eV, which is consistent with the data of the flat-lying F16CuPc on HOPG.40,44 Furthermore, based on the orientation-dependent IP of the ordered molecules, the observed IP of F16CuPc molecules varies from 5.8 to 6.4 eV in the lying and standing F16CuPc films,40,44 respectively. In our case, the measured IP of the monolayer F16CuPc is around 5.9 eV, which is very close to that of the lying F16CuPc. The small discrepancy is due to the uncertainties in the estimation of the HOMO onset of the monolayer F16CuPc on the graphene. Figure 6 clearly demonstrates that monolayer F16CuPc molecules play an important role to affect graphene work function, which leads to a vacuum level alignment with the deposited pentacene. The most important observation is that the deposited pentacene molecules form thin films with molecular packing in an upstanding geometry, which results in a lower hole injection barrier due to the splitting of the HOMO peak.45 Finally, it should be mentioned that, in device fabrication, it is impossible to make graphene as clean as in vacuum conditions. There will be molecules adsorbed on graphene surfaces as ″impurities″. To clarify the influence of an impurity on graphene upon the following organic molecule growth, we also studied the pentacene films on ″dirty″ graphene film. Such graphene films are under ambient conditions for long times before they are transferred into a vacuum chamber. Interestingly, the valence region spectrum indicates a lower hole injection barrier of 0.6 eV than is the case for clean graphene. On the other hand, the dirty graphene can be considered as a passivated graphene, in which oxides or residual hydrocarbons exist on the surface. Therefore, there will be even weaker interaction with deposited organic molecules than for clean graphene. The angle-dependent NEXAFS measurements demonstrate that the pentacene molecules are formed with molecular packing of an average tilt of 45°. The change of the tilt angle can be assigned to the impurity influence on both surface roughness and the change of surface potential, both affecting the interplay between the molecule and the surface.

Another significant feature of the pentacene valence spectra is the very low hole injection barrier, about 0.3 eV, whereas it is 0.7 eV on bare graphene. Furthermore, compared to spectra in Figure 1, the valence band of pentacene film on modified graphene shows a completely different spectral feature in both the energy position and the relative intensity. It should be mentioned that there are no discernible signals from the substrate (F16CuPc and graphene) at 3 nm pentacene film. Upon further pentacene deposition, as shown in Figure 4, no obvious change of the frontier valence region spectral feature was observed. The change of the valence band spectral feature of pentacene on F16CuPc-modified graphene can be further illustrated from the corresponding NEXAFS measurements since previous studies indicated that the difference of the valence band spectrum of pentacene films results from their molecular stacking.41,42 Figure 5 presents the NEXAFS C Kedge of 3 nm thick pentacene film on F16CuPc-modified graphene as a function of the angle of the incident beam relative to the sample surface. In the inset of Figure 5, the NEXAFS N K-edge of thin F16CuPc film were shown with the beam at normal (90°, dashed line) and at grazing incident (10°, solid line). It clearly indicated the F16CuPc molecules are flatlying on the graphene surface. Such orientation is consistent with STM observation,20,35 in which F16CuPc molecules form hexagonal-like patterns on the monolayer graphene. It should be mentioned that there is still remaining π* intensity at normal incidence, even with F16CuPc molecules completely flat-lying on the graphene. This discrepancy can be understood, e.g., by weak in-plane polarized transitions in the energy range of the π* resonances, as observed in many phthalocyanine molecules on different substrates.43 In the deposited pentacene film, the NEXAFS spectra show a strong π* resonance at normal incident (90°), whereas the highest σ* resonance was reached at glazing incident (10°). The quantitative analysis of the change of the π* intensity with different incident angles yields a pentacene molecule orientation with an average of 89° ± 5°. Therefore, the change of the valence band spectral feature in the bare graphene and F16CuPc-modified one can be assigned to the different pentacene molecular stacking with the former tilted on the surface at an angle of 29° and the latter being up-standing on the surface. The up-standing molecular geometry in pentacene film will lead to a stronger π−π overlap between the pentacene molecules and enhance the intramolecular interaction. Combining with the lower hole-injection 3973

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(7) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308− 1308. (8) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; et al. Graphene-Based Liquid Crystal Device. Nano Lett. 2008, 8, 1704−1708. (9) De Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou, C. Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics. ACS Nano 2010, 4, 2865−2873. (10) Pang, S.; Tsao, H. N.; Feng, X.; Muellen, K. Patterned Graphene Electrodes from Solution-Processed Graphite Oxide Films for Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 3488−3491. (11) Di, C.; Wei, D.; Yu, G.; Liu, Y.; Guo, Y.; Zhu, D. Patterned Graphene as Source/Drain Electrodes for Bottom-Contact Organic Field-Effect Transistors. Adv. Mater. 2008, 20, 3289−3293. (12) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (13) Lee, C.-G.; Park, S.; Ruoff, R. S.; Dodabalapur, A. Integration of Reduced Graphene Oxide into Organic Field-Effect Transistors as Conducting Electrodes and as a Metal Modification Layer. Appl. Phys. Lett. 2009, 95, 023304. (14) Yamane, H.; Nagamatsu, S.; Fukagawa, H.; Kera, S.; Friedlein, R.; Okudaira, K. K.; Ueno, N. Hole-Vibration Coupling of the Highest Occupied State in Pentacene Thin Films. Phys. Rev. B 2005, 72, 153412. (15) Koch, N.; Vollmer, A.; Salzmann, I.; Nickel, B.; Weiss, H.; Rabe, J. P. Evidence for Temperature-Dependent Electron Band Dispersion in Pentacene. Phys. Rev. Lett. 2006, 96, 156803. (16) Chen, W.; Huang, H.; Thye, A.; Wee, S. Molecular Orientation Transition of Organic Thin Films on Graphite: the Effect of Intermolecular Electrostatic and Interfacial Dispersion Forces. Chem. Commun. 2008, 4276−4278. (17) Bussolotti, F.; Han, S. W.; Honda, Y.; Friedlein, R. PhaseDependent Electronic Properties of Monolayer and Multilayer Anthracene Films on Graphite [0001] Surfaces. Phys. Rev. B 2009, 79, 245410. (18) Götzen, J.; Käfer, D.; Wöll, C.; Witte, G. Growth and Structure of Pentacene Films on Graphite: Weak Adhesion as a Key for Epitaxial Film Growth. Phys. Rev. B 2010, 81, 085440. (19) Huang, H.; Chen, S.; Gao, X.; Chen, W.; Wee, A. T. S. Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene. ACS Nano 2009, 3, 3431−3436. (20) Wang, Y.-L.; Ren, J.; Song, C.-L.; Jiang, Y.-P.; Wang, L.-L.; He, K.; Chen, X.; Jia, J.-F.; Meng, S.; Kaxiras, E.; et al. Selective Adsorption and Electronic Interaction of F16CuPc on Epitaxial Graphene. Phys. Rev. B 2010, 82, 245420. (21) Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal-Organic Interfaces: New Experiments and Assessment of the Field. Mater. Sci. Eng. R 2009, 64, 1−31. (22) Koch, N. Energy Levels at Interfaces Between Metals and Conjugated Organic Molecules. J. Phys.: Condens. Matter 2008, 20, 184008. (23) Braun, S.; Salaneck, W. R.; Fahlman, M. Energy-Level Alignment at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 2009, 21, 1450−1472. (24) Moench, W. Slope Parameters of the Barrier Heights of MetalOrganic Contacts. Appl. Phys. Lett. 2006, 88, 112116. (25) Vazquez, H.; Dappe, Y. J.; Ortega, J.; Flores, F. A Unified Model for Metal/Organic Interfaces: IDIS, ‘Pillow’ Effect and Molecular Permanent Dipoles. Appl. Surf. Sci. 2007, 254, 378−382. (26) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Orientation-Dependent Ionization Energies and Interface Dipoles in Ordered Molecular Assemblies. Nat. Mater. 2008, 7, 326−332.

Considering the molecular packing in the devices, the closer tilt angle to up-standing orientation, and the better performance, the dirty surface is favorable for the device performance, and such results are consistent with pentacene on the transferred graphene interface, where the existence of polymer residues, like polymethylmethacrylate, can lead to a high field effect mobility in the pentacene device.32

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CONCLUSIONS We presented detailed interfacial studies of pentacene molecules on the graphene/SiC substrate. Pentacene molecules were grown in a tilt fashion with an average angle of 29°. The molecular tilt angle shows no obvious change with increasing film thickness. Even in the first monolayer, the pentacene molecules are formed in a favorable (022) plane growth rather than flat-lying packing on the surface. Upon monolayer F16CuPc modification, the work function of graphene was increased about 0.15 eV, for further deposition of pentacene molecules, and a vacuum level alignment was reached. The most important note is that the F16CuPc monolayer strongly affects pentacene molecular stacking geometry, in which an upstanding pentacene molecule film was formed, and a lower hole injection barrier of 0.3 eV can be achieved. Such an effect will be very useful for the corresponding graphene-based electronic device fabrication.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professor Mats Fahlman for useful discussions. X.L., A.G., and A.V.F. acknowledge EU support through ELISA for the stay at BESSY. A.G. acknowledges an APART fellowship from the Austrian Academy of Sciences. T.P. acknowledges support by the FWF project I377−N16. This work was supported by the FP7-Energy-2010-FET project MOLESOL (Contract No. 256617).

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REFERENCES

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dx.doi.org/10.1021/jp3103518 | J. Phys. Chem. C 2013, 117, 3969−3975